I'd guess that some older fans (myself included, to a point) have become fed up with the franchise by this point, because it hasn't changed in any really significant way since the first games were released, other than graphically. The tweaks they make between generations are so minor that they add barely anything to the game; you might as well replay one of the previous generations, it's much the same thing. You can only play something so many times before you get bored of it, which might be why people are less than fond of B/W, especially if they've been with the franchise for a long time: they've already played the games four times before, pretty much.
The same could be said of previous generations; people come looking for something fresh and exciting, and they don't really find it, because Pokemon doesn't really need to change to sell, so it doesn't.
I suppose designs of Pokemon probably play a part as well, but then, there have been Pokemon in each generation that people haven't really liked, so I wouldn't have thought that'd be a huge factor. Some you'll like, others you won't and, even if you don't like any of the fifth generation, there is still the first four...once you've finished the game. That restriction - being stuck with fifth gen Pokemon for the entire game - might have something to do with it. I know I was extremely aggravated when I found that I wouldn't be getting any of the original generations until I'd finished the main story.
In short, I'd say people who hate B/W probably have the same reasons for hating the previous generations, and in a lot of cases it probably won't be hatred: they're just bored with a franchise that never changes. I can't honestly say I blame them, all things considered.