Many things, but as I don't have much time I'll keep it brief:
1. Everyone liking Pokemon. Like, I mean everybody. Now, I have to come onto Smogon just to find someone to play with :( (but it is a great thing in a way because this is how I found the site!)
2. The overall feel. I haven't had an experience with a Pokemon game since Red, Blue and Yellow that has made me feel the same way I did. It has a lot to do with nostalgia, but mostly because of the feelings of discovery and curiousity that they have failed to instill in later generations. I'm not really sure how they could fix this, maybe its partly my fault for trying to find out everything about the game before it comes out. What they can fix though, is the design of the pokemon. I think no generation compares to the first in terms of pokemon design. I think to make the next generation really memorable, they have to draw back on the art from the days of red and blue. I think this starts with the starter pokemon-- no starters in generation 2, 3, or 4 could hold a candle to Charmander, Bulbasaur, and Squirtle (Generation 4 was on the right track, until they evolved them). I loved Generation 4 (don't get me wrong! Generation 3 was my least favorite), so I think they are on the right track for Generation 5 and have a shot of returning the memorable aspects.