Sure.

Besides the better UI, it was definitely small things like one- you get 3 icons in the app screen as opposed to 4 on the N9 (higher res of course) and you never got away from that "old phone" look that has been present even since my 5800XM.
Getting to the options or new messages was a pain because you had to get back to the home screen or had to delve into settings to change something simple.

Messages- new texts either would open up in the conversation mode or in one text by itself and then I would have to exit out of that, go into conversations, and get the other ones. The N9 tells you how many and takes you right to the conversations (it doesn't have this split interface stuff like the N8) and I can see easily what's new and what's not and it *always* has the message in the context of the entire convo (like the N900.)

Email- I almost never heard my email go off on the N8 and there was no visual indicator unless you had your mailbox widgets on the homescreen. For a guy with 5-6 email accts, that takes up one entire screen by itself. It was crazy to waste that space with just mail whereas even on the N900 you could have ONE widget that displayed them all. On the N9, I get a systemwide indicator of new mail and it immediately displays it at the top of the feed. Too easy. That and the organization and look is a million times better than the N900's Modest.

Other than that, it overall is a more united, more pleasing UI experience and has the small touches that make it an awesome phone. Swipe is the best interface idea to come about since I don't know what. I mean, I know it sounds like I didn't like the N8 but I honestly did (especially for the camera) but it was still in this old mode of thinking wrt apps and UI (and IMO Android and ios are no better.) I'm sure Belle will make it better but to me there really isn't any comparison anymore. I might have to buy another N9 for when this one eventually dies. It really is that good.

Hope that helped. Was a bit longer than you asked for but I'm still crazy about this phone.