I wasn't surprised that they made older-gen Pokemon available in B2W2; since Black and White were basically introductions to Unova, a brand new Pokemon world, it seemed reasonable that we could only use entirely new Pokemon for the main story. However with B2W2, I think people would've been a bit tired of that, and so the 'Pokemon migrating' excuse works perfectly well.
I was surprised at the sheer amount of previous-gen Pokemon that were made available in B2W2- I still can't get past the fact that there's 300 Pokemon in the Regional Dex alone. And I pretty much love ALL the Pokemon that were included, it's like GameFreak just knew what Pokemon I'd want and said 'Yeah, let's put them in the first few routes!'. So I was very pleasantly surprised about that. :)