There's been many reasons why people have been upset about Ash in different seasons from using non-evolved Pokemon, to voice acting/dialogue, to being less of a character than they are capable of making him. Honestly, I think it simply boils down to the writers not really knowing how to make Ash change and still be popular at the same time. I'd go as far as to say they don't even know what their audience really wants from Ash because of how mixed opinions are about so many aspects. One things for sure though, they need to start by being consistent with their foundation. What are the core values of Pokemon and how does Ash, as the main character, help express those values? When does Ash utilize trademarks of his character such as rashness, determination, and thinking outside the box, and how can that be balanced more effectively? What types of friends, both human and Pokemon, can surround him so they compliment each others personality and development? If they can come up with a solution to fit the answer to questions like these, Ash will hopefully have a better standing in the fandom.
I am the current leader of Save Our Voice Actors, a Pokemon fan group supporting the voice actors in the anime dubbed by 4Kids (seasons 1-8). Our goal is to provide The Pokemon Company international with encouraging feedback in support of returning the main cast members. If you're interested, please visit us at facebook.com/saveourvoiceactors