I think they made collecting much more fun, easy, and useful (unless you were just doing it for achievements)in AC2, with being able to look up feathers by district, fitting it in the story, and actually giving you stuff if you did it.
I recently went back and got all the flags in the original AC since I had so much fun with the feathers, and I gotta say, if they put collectibles in a big connecting area like the Kingdom again, they need to outline the buildings and the different levels of terrain. It was usually fairly easy to follow a guide to find flags in cities, since you can remember, "Ok, it's at the edge of this building." I, like many other people, got stuck at 99 flags, and was looking everywhere for the last one (which I eventually found).
Also, if there are collectibles in the next console AC, I think the items should be numbered, so it's easier to know which ones you already have. You know how in AC2 in the DNA there was a different picture for every area's feathers? I'd think it would be cool to have cool pictures for every item as well, like you click on the city and you can browse all of them. Leave you opinions in the comments, please!