i think i would have to say trade evolutions. now its not so bad if u have wifi but i remember playing gens 1-3 and wanting a kingdra and a few others and i never had friends who played pokemon and i never wanted another gameboy at the time just to do a trade evolution.
other than that i would get rid of bronzors. they annoy me so much -_-
I'm walking fine then "A WILD ZUBAT APPEARED" and in 4th gen the animations were so slow it made it even go on for longer. At least Black and White will make the Zubat's animated.
I would get rid of the event of May being replaced. Plus, Legendary Pokemon, I hate battling people and having a Mewtwo killing all my party. Allowing people to overpower themselves is never good in multiplayer games.