Well for one, since Gen V mirrored Kanto in some ways, and with the lack of any previous ge Pokemon, they decided to go for some kind of weird half-reboot.
With no contests in Unova, and no time for a Sinnoh Frontier series or HGSS-inspired Johto series, we ended up losing Dawn, Brock, and Paul.
And then they blatantly ignored all the potential material they could have gotten from Gen V. Iris, who goes from being a Gym Leader to Champion, is instead just a dragon trainer, missing so much potential character development. Team Plasma, the evil Team with the most story focus, is shoved aside into a brief arc, and we lose all potential character development Ash could have gained from having to confront Team Plasma's beliefs and potential interaction with N. Cheren and Drayden, two Gym Leaders who play heavy roles in the story, appear briefly while Ash has a joke of a battle with Roxie, the least developed new Gym Leader. PWT could have been a great reunion period where companions from the entire anime all returned, but since Dawn was written off after all the charater development she received, PWT was just a random tournament arc used for Dawn's cameo.
Between BW and BW2, there was great potential for not one, but two Unova journeys. Instead, we got one garbage series. If they had followed the success of DP, the anime could have reached it's peak.Understandably, I would not have been surprised or terribly angry if Ash still lost the league and continued traveling, but if BW anime was done properly it could have been the perfect finale for Ash.
