After receiving your Pokemon from the surprisingly hot professor Juniper, your quest to catch em' all begins...again...
(Admit it guys, you'd let her handle your pokeballs. XD)
Team Plasma plays a much bigger role in the game, similar to Galactic in D/P/Pt. However, while Cyrus wanted to re-create the world using Legendary Pokemon, Team Plasma wants to force everyone to release their Pokemon, considering it cruel that they're confined to Poke balls and raised to fight...Yet they use Pokemon in poke balls themselves.
There are other changes that have made the game much more practical. Pokemarts and Pokemon centers are now located in the same building, and TMs (items used to teach pokemon new moves) can be used as many times as you want. There are less HMs (Moves used outside of battle to clear obstactles/Travel) however, but that can be considered an upside since they take up move space anyways. One problem I have is that the VS seeker is gone, and wasn't replaced with any other way to re fight trainers. However, there are certain locations in the game where a decent # of trainers can be fought every day after beating the Elite four, much like the Saffron Dojo or the Survival area in previous games. You can also fight Cheren on a daily basis once you've beaten the Elite Four
Wild Pokemon are much higher level in this game than in previous ones. So once you've fought all the trainers you can a day, it's worth your while to hit the grass. Not to mention Juniper gives you a wonderful item called a Lucky egg after the 5th gym. (Give it to a Pokemon to hold and it boosts EXP earned in battle.)
Either way, there are new towns, returning pokemon from the older games and more trainers to fight even after beating the Elite 4.
With Legendaries though, you'd think once God has been done (Arceus) the others are just a downgrade. They still manage to make them interesting however. Zekrom and Reshiram still have a decent mythology (I'll say nothing else at risk of ruining the story, since this game is more story driven than previous games.
If you are a nostalgia freak, remain calm. you can Migrate Pokemon from any of the DS games after beating the Elite Four. (you'll have to migrate GBA Pokemon to another game first, then send them to black and white with another DS via wireless. Luckily, my sister has a DS so I don't have to worry about this.) If you're lucky enough to have a friend who's completed that game, they can trade you any Pokemon they want, it simply won't show up in your pokedex until after it's upgraded to National mode.
I also like that they experimented with type combinations more (Some examples are Bug/Fire, Ghost/Ground, Electric/Flying, Dark/Ground, Bug/Electric, Ghost/Fire, Dark/Steel, Dark Dragon.)
For those who thought TL;DR: Pokemon Black/White is great contribution to the franchise. For older players, it gives us another fix and for newer players it manages to be as accessible than the older games, if not more. If you've ever enjoyed a Pokemon game, I can recommend this one as much as any. Overall, 8.5/10