I think the two main reason people don't like him is...

1) He's not really important to the plot, even Sakaguchi and Kitase admit he's not important but kept him in because they liked him conceptually. Also, the wild child third-person speak is a bit of a love-it-or-hate-it type.

2) He is a giant Guide Dang it character. He's a pain to acquire all of his Rages, to understand how they work you need either to have a guide or do a tone of trial and error gameplay and he can be very time consuming to most newbie players. With that said, he's probably one of the most broken characters in the game hence why he is the only character to get nerfed in subsequent games.

Despite all this I love the little guy as well. He has a great character theme and he's funny.