I've probably been ninja'd an infinite amount of times in this thread, but oh well.
First, I extremely love the battle system. Not only have the graphics and animations been increased, but Serebii has a touch-screen picture, and it is definitely a lot more detailed. I don't know if Serebii confirmed this, but I'm hoping you can see what stat boosts/drops you and your opponent have. So, if you are on turn 15 of a battle, and you forgot how many times you used Swords Dance and how many times your opponent switched in Gyara, you can easily check your +Atk level. That way, it's one less thing to keep a track of.
items being able to change the weather
I don't think anybody has confirmed this, and imo it's just a rumor for now. That would be extremely cool in-game, though, as you don't need to shovel Rain Dance or Sunny Day on all of your Pokemon. A shame it most likely won't work in WiFi, though. On that note, I absolutely love the Random Battle system for WiFi. There is always a scare of hackers, but it has to be really fun randomly battling people without a need for contact or FC exchange.
Out of the new Pokemon, my absolute three favorites are Chiramii, Gear and Reshiram. On that note, the legends had some awkward typings. I'm so used to the Pokemon being color-coded for their type (red = fire, blue = water, black = dark), so when I saw a white Pokemon having a Fire sub-type and a black Pokemon having an Electric sub-type, I was thrown for a bit of a loop. I guess that makes sense if their signature moves are "White Fire" and "Black Lightning", or something to that effect.
With my logic, the third game in the series will be Pokemon Gray, with a Gray legend being Dragon/Ice or Dragon/Water. I hope for the former, since we already have both Kingdra and Palkia for the latter typing, and we don't have any Pokemon for Dragon/Fire or Dragon/Electric. On the topic of types, I really hope they continue the Dragon-type psuedolegends. Gen II was the only one to not have that, and they swiftly changed it back by including 'Mence.
One minor change I hope they make is re-instituting the bike from DPPt. Y'know, where you could switch gears between fast and slow. Only, except for fast and slow, they make it Mach and Acro (Gen III nostalgia). That way, you still have the fast bike you know for Gen IV, but you also get that trippy bike from Gen III. That also opens up a lot of new "puzzles" and new areas you can visit.
And one last thing, I hope they get rid of "Acid Rain". It's not a major change, but it really screwed over weather teams and Pursuit-users in WiFi.
I cannot wait for BW to come out, and I need to wait until Spring of '11 since I can't read Japanese. I'm eagerly awaiting the new metagame, and I hope it doesn't change too much. Although, the switch from ADV to DPPt was pretty big, so it's a false hope.