Level-up movesets in particular have been edited for 600+ Pokémon. A lot of Pokémon have been given their Dream World abilities as an option, where applicable and/or useful.

This can be a number of things, including experience rate, typing, base stats and abilities. The level curve takes full advantage of the new experience system in Black and White. While not mind-crushingly tough, they're sure to provide more of a challenge than the original games were able to.

This is unaffected by the version chosen to play encounters are the same in both. As aforementioned, the hack allows you to catch all 649 Pokémon in some way or another.