I've only played the First  Blood intro quest (twice), but I've read through all the game material and cards. Some thoughts:

1. I miss having actual coin tokens. In fact, we'll probably just use the tokens from 1E for our 2E campaign.
2. We'll probably use the tuck boxes from RtL or SoB for storing our campaign stuff
3. I miss the breath template
4. I really like the new movement rules for large creatures.  Although maybe not as realistic as 1E, it certainly adds a whole new way of handling big monsters. I think this will prove to be a big + for the Overlord.
5. I love relics as much as the next guy, but it seems like the campaign gives out a lot of them. So much that they almost feel like "Gold Level" items from 1E rather than truly spectacular or hard-to-achieve items.  By the end of our campaign I wouldn't be surprised if there were 4 or more relics in play, which diminishes the "special-ness" of them IMO. 
6. At one point in the story prose, the Overlord literally twirls his mustache and another time says "Hahaha".   

I am really excited to see the conversion kit. As somebody who owns ALL of the 1E expansions, it will be great to be able to throw down a demon or dark priest or ice dragon.