They reset Ash every region but never as much as they have in BW.
Before they just make him go to a new region with Pikachu so he has to catch new pokemon as a kind of reboot for every new region.
But in BW they changed his character design (they eye thing and he is smaller than in DP) and they also made pikachu lose to a lvl 5 starter which can be explained with the Zekrom plot point but a few episodes later they made pikachu lose to a water type from the first gym while he was at full power. I could also go on about his lack of knowledge of things he has learned before but that would take to long.
And another thing is his age. Before BW it was left to speculation by the fans if he has aged at all or not but than in BW they blatantly said he was still 10 slapping all of the people that made systems to see how old chars are (me included) in the face.