I think they should not have made a Dream World. The hidden abilities could have been gotten another way, though I don't know what that could have been.
I never used the Dream World, because it felt like it was not really a part of the game. They dedicated an entire part of the C-Gear to that one feature, when they could have made many more features accessible through the "Online" separator, and you had to go to this website and do things there (Game Sync is related to the Dream World, right?).
Part of the problem is not everyone enjoys online minigames (like me). A *lot* of sites nowadays (and even some time ago, too) made minigames their chief feature of the website (e.g. Cartoon Network, Adult Swim, Facebook, Yahoo, etc.). It feels like Nintendo is just trying to grab a piece of that pie along with all the other minigame-playing sites.
Pokemon X/Y seem to have the right idea. Make more features part of the game, e.g. Pokemon Amie, the rumored seeking feature that I keep hearing about, etc. I have no problem with the GTS website, because there are no real minigames and you don't have to go there if you don't want to - you can chiefly use the GTS from the game directly.
I think they could have done much more than another game-linked website. Especially since I just found out there are exclusives that only come from the Dream World - Pokemon, for example.
In essence, They only have so much ROM space to work with and now that part will just be dead code.