OK, so, there IS an Imperial Society.
It is a society populated and practiced by nobles and higher echelon citizens with the wealth or power to engage in interstellar activities.
It is derived from the fashions of Holy Terra and the High Lords, both of which act as the 'cutting edge' of fashionable society, and trickles down through 'core' worlds, to eventually outlying worlds ('core' and 'outlying' being defined by their cultural proximity rather than their physical location(?)).
It is a society, with attributes that can be aspired to. Those planetary 'bigwigs' who accrue enough wealth / power aspire to enter that society.
Other 'local' (planetary) people who will never be able to accrue enough to actually participate in the interstellar society, still know about and understand it, and will look to make their local culture / society emulate this Imperial culture as much as they can.
Imperial society is therefore a paragon; a model of 'virtue' and a model of 'how to be a good Imperial citizen'.
So the question still remains, what are the details of this society?
Location Where is the Imperial Society practiced? Are their 'core worlds' where Imperial Society covers the planet? Is it a society that exists only on Holy Terra and on interstellar locations like trade ships, space stations, etc.?
Descent Is Imperial Society and elitist group like hereditary nobility? So belonging to Imperial Society requires a birthright? Or is it open to anyone with aspiration and the resources to enter it.
Language What language is used by the Imperial Society? High Gothic presumably? Or does it have its own exclusive language as an elitist barrier?
Customs and culture What customs, activities, and cultures define Imperial Society. How would someone have to or wish to act to claim or show that they belong to or understand Imperial society?
Beleifs What beleifs does Imperial Society engender? Presumably this is where the Imperial Cult or Creed would come in, but what are those beliefs?
Material culture What sort of gear would a person wear or use to express their affinity for or membership of Imperial Society?
'A wise man doesn't know how it feels to be thick as a brick' - Ian Anderson
'One of the advantages of being disorderly, is that one is constantly making exciting discoveries' - A.A.Milne
'Beware of the man, who's god is in the sky' - George Bernard Shaw
"The secret we should never let the gamemasters know is that they don't need any rules." - Gary Gygax
Location: "Imperial Society" is the society of the nobility, and is practiced wherever they congregate.
For example: sector capital worlds, the upper spires of most hives, portions of pleasure worlds, the capitals of prosperous agri-worlds, and so forth. Keep in mind, however, that the more distant from the centers of power and fashion, the more out-of-date and provincial the version of Imperial culture practiced.
Holy Terra might be said to be dominated by the Imperial culture, but in truth Terra is as much influenced by the distinct subculture that holds sway within the ecclesiarchy. Trade ships and space stations have a common culture within a given sector, but it is not Imperial; it is, rather, the distinct culture of spacers, influenced by the Machine Cult (as they depend on machines constantly) and rife with strange superstitions born of proximity to both void and warp. Stations also likely contain enclaves of the cultures of nearby planets, made up of those hardy souls who would risk a life in space for the hope of profit and a chance to slip the bonds of the life they were born into.
Descent: Strictly speaking, one need not inherit a place in Imperial Society. However, a lack of illustrious ancestry is one of many things that will reduce your status in the eyes of your peers and betters.
Imperial Society is obsessed with status; sector nobility (those whose influence touches several worlds) look down upon planetary nobility (those whose power is confined to a single world,) who in turn look down upon regional nobility (those who govern a single city or portion of a hive, controlling the lives of a paltry few million souls.)
The origin of one's influence is equally important; families whose power comes from longstanding ties to the church disdain those merchant princes whose power comes from the trade and transport of mere material goods. Families with a history of service as high-ranking officers in the Imperial Guard sneer behind their hands at the priests, and so on.
Therefore, if you wish to be fully accepted into Imperial Society, you must either have personal achievements that outweigh your low birth (as any formal party is enlivened by the invitation of a genuine hero) or marry into an established family; preferably both.
Language: The languages of Imperial Society are High and Low Gothic. Formal events are conducted in High Gothic. Normal conversation is conducted in a mixture of the two; primarily Low Gothic, with frequent quotations or borrowed words from High Gothic. This is done to show the speaker is cultured, educated, and generally better than you, like someone with a classical education who constantly quotes in Latin.
Individual families, however, may use a different language for internal matters. For example, House Cassius, a wide-spread merchant family, uses Stone Tongue, the dialect of their homeworld Plutarch's Rock, in private conversation as a simple precaution against being casually overheard.
Customs and Culture: The heartbeat of Imperial Culture is its parties. Here, over fine amasec and off-planet delicacies, status is evaluated and re-evaluated, information is disseminated by the medium of rumor, one can do a headcount to determine who has killed who in duels, and, very importantly, you can see what people are wearing.
Personal ornamentation is very important, not merely as plumage but as communication. A discrete symbol shows that your family takes pride in its piety and enjoys the power that comes from an uncle being a cardinal. A military dress uniform implies that you are a war hero just stopping by for a chat and a drink before rushing off to save the Imperium. An oddly curved earring informs those in the crowd who know what to look for that you are a cultist of Slaanesh, and might be of assistance should they find the current party boring. Two augmetic fingers advertise that you are Commisar Cain, and any noble ladies who wish to pull you into an alcove for a private chat will not be dissappointed.
To be accepted into Imperial Culture, one would need fine clothes, the ability to speak both High and Low Gothic without an embarrassing accent, a working knowledge of etiquette so you don't shame yourself by choosing the wrong fork or telling a bawdy joke to the baroness, and an introduction by someone already accepted by the culture.
Beliefs: All in the Imperial Society follow the Imperial Creed, at least in public. It would be social suicide not to. In private, they run the gamut from the truly, deeply pious, through the spiritually apathetic, to those touched by chaos. The worship of Nurgle is rare among the nobility. Khorne has a following among some families of a military or murderous bent. Slaanesh and Tzeench are quite popular, among the decadent and ambitious respectively.
It should be noted that among vamilies engaged in bloody vendetta, "having a spiritual awakening" and becoming a monk or nun is an acceptable way to escape the cycle of assassinations by abandoning all claims to power.
Material Culture: Fine clothing is a must. Xenomesh armor may be worn to protect against assassins, as it is far less crudely obvious than other armors. Grooming is of paramount importance; beards must be neatly trimmed, hair well coiffed. For personal protection, a high-quality las-pistol or two may be carried (I favor Palantines;) stubbers are sometimes regarded as crude tools of the violent lower class, and carrying a bolt-pistol openly announces your intent to cause mayhem. If you don't have a personal servant or personal servitor, for heaven's sake don't let anyone know. A small amount of fine jewelry is encouraged as a discrete display of wealth.
And we always have the unifying Imperial Battlefleets and Adeptus arbites. ;)
=][= Without signature
Personally, as much as I dislike most of Kevin J Anderson's work, I think that possibly the best model of how Imperial society would be can be found in the Dune-verse faufreluches and Landsraad, and unfortunately, most of the detail on them appears in the Prelude to Dune trilogy by Brian Herbert, and the aforementioned K.J.A. It's a pity really, I much preferred the stuff that was merely hinted at by Frank Herbert in Dune.
Another example might be the court of Empress Lionstone IV in Deathstalker, although part of me gets the impression that that was really just the court of the Padishah Emperors in grotesque parody.
I'd suggest that much of the time 'society' would speak Low Gothic for day-to-day business, maybe ordering servants, but for any kind of social function would be using High Gothic, if not High Formal, or some other equally snooty dialect.
As for where 'society' would meet, I'd say probably the best answer would be on those worlds which have been given a 'high' administrative, religious or military status; the nodal sytems of the military/bureaucratic response, essentially, plus major Ecclesiarchy worlds, and also the 'paradise' worlds that run as major/exclusive pleasure resorts. However, I'd also note that those are just my estimations of where you would be most likely to find them, as it has been noted that society is wherever the members of it choose to congregate.
The membership is probably determined semi-hereditarily: those with particular power in an acceptable sphere would be at least honorary members of society (so, Lords Militant, Planetary Governors, Archbishops and higher, senior Administratum adepts and so on), as would the owners of starships (as opposed to spaceships, which I would define as non-warp-capable), and indeed, the captains of said starships. Holders of Adeptus Warrants, Charters and Letters of Marque would form their own hierarchy within society, with the holder of a Rogue Trader's Letter of Marque would be near the top, probably taking precedence over everyone below a Planetary Governor or Imperial Commander.
Other thoughts when I can wake up enough to think straight...
Then the Prophet spake 'Frak this, for my Faith is a shield proof against your blandishments!'- Alem Mahat, Cain IV:21
OK, so, obviously, every world in the imperium, to a greater or lesser extent, maintains a unique planetary society.
But is there an 'Imperial Society'? An overarching, interstellar, society that defines the imperium, perhaps providing heroic, moral and societal paragons for the various disparate societies to emulate and aspire to? If so, what is it like? How is it constituted, structured, maintained?
The example i'm thinking of is Ancient Rome. The various Roman provinces all had their local cultures, customs etc. Gaul as different to Numidia. Egypt was different to Iberia, etc. However, they were all Roman; they all adopted Roman culture, religion, materials, building styles...Egyptians and Gauls alike wanted to 'be Roman' (not everyone of course, but many, many did).
So is the Imperium similar?
I would think the Islamic conquest would be a better example. In this case Arabic language, culture and a unifying religion was imposed upon each country. It wasn't a melding as the Greeks tended to do or as much a dualistic one as I see the Romans - Romans and 'everybody else who are barbarians'. Instead you virtually have every culture conquered, with the exception of Persia/Iran, where Arabic became the dominant language and culture with Islam the dominant religion. There are some local variances but they aren't that major.
Heretics just aren't going to kill themselves.
I guess we have already established that there is a Imperial society. The most important tool when it comes to define and understand culture is language. Imperial society has a language and that is high Gothic. That is used not only on formal occansion but also in poetry and litterature. That means that everyone in the Imperium has agreed on how to use that particular language and it´s concepts. Otherwise metaphores and other narrative acrobatics will be useless leaving litterature and poetry really boring. Every societies culture lies within it's litterature and there is were you can find the values and ethics that are valid in that culture. To spread culture you have to spread it's litterature.
Fashion and other trends can be important in certain moments but since they are fluid and always changing they carry no fundamental culture within them. Unless there is certain elements that always comes back. The black tie is a good example of fashion becoming culture. Otherwise it is mostly used by sub-cultures or classes within the society.
Since the Imperium is such a large place the imperial society probably is very abstract and composed of few but important features. The are a number of values like sacrifice and obedience that are important. Maybe social behaviors that tells the one you meet were you come from and of course how you use high gothic.