Wow, and I skipped Unova, thinking it was a waste of my time. Meanwhile, some of you say it was the best lol. (i have no reasoning, just didn't see it hyped enough to seem worth it to go back to it, maybe i should now, if no remakes are announced)
Black/White do not feature any older Pokémon until after the end credits, as the focus was strictly on the 156 new species; a lot of people didn't like this, but in my eyes it was refreshing as you had to experiment, find out what each new 'mon was all about, instead of relying and coasting by on old favourites. It also helps that Unova contains many of my personal favourite Pokémon.
BW2, meanwhile, brought back a lot of older Pokémon and mixed them with the new, making it a more standard romp in that regard.
Personally, I did like N's story and character progression throughout BW, though there isn't much to do in those games after fully exploring the region.
BW2 drastically improves on that front, though, by adding awesome features like Challenge Mode (sadly only available by beating Black 2, at which point you can transfer the Challenge Key to another copy of BW2 - or use an Action Replay code to unlock it immediately), Pokémon World Tournament (where previous region's Gym Leaders and Champions can be fought, alongside Red of course), Black Tower and White Treehollow (mazes, where you need to find a specific Trainer who will unlock the door to the next level), and Pokéstar Studios (short movies you can 'act' - battle - in, and what you do can influence the plot, resulting in some weird and funny endings).
There's also Hidden Grottoes, which can rarely contain specific species with their Hidden Abilities; a nice small bonus feature, that.
Also, back when the external Dream World was still accessible, you could change your C-Gear's skin, grow berries, and get Pokémon with their Hidden Abilities through it; BW2 also had a basic achievement system known as "Medal Rally", and some Medals were exclusively gotten through Dream World activities...which meant that once DW shut down, Medal Rally could no longer be completed...ugh. Also there's no other way to grow berries in Unova, so that sucks too.
Oh, and while I do love BW2, its Champion is unfortunately not the same one as in BW; instead it's a rather minor character who really did not do much of anything throughout both BW and BW2, which was exceptionally disappointing.
Still, if you finish BW and then link it to BW2 through DS Wireless, you can unlock some extra flashback scenes involving important characters like Gym Leaders and the Champion, as well as being able to catch some of N's Pokémon and able to battle Bianca and Cheren with their original BW teams.