It's mostly the fact that he can't really sing, carry a tune, or find the pitch to save his life. I saw them on SNL just a year or two ago and he was so amazingly bad and embarrassing my wife and I had to switch the channel. Worse than anyone I've heard totally flailing on my local scene in years. Completely out of tune, out of time, and croaking more than vocalizing. It was pathetic. He made some decent records a few decades ago, but talk about resting on you laurels.
I'm surprised about the Chris Cornell comments. I could see not liking his style, maybe, but he's a technically gifted vocalist, everything Vetter is not, except perhaps a crazy egotist. I think they both suffer from massive untreatable cases of LSD. Anthony Keidis: yeah, B+ for effort, but he needs a new rhythmic pattern for his tuneless chanting. Too bad because his vocal quality and energy are a good match for the Peppers, I just don't think he was able to make the transition to their new style as well as the other members of the band. Axl Rose: so annoying! And so much like a wanker version of Ethel Merman. Really have always felt he ruined GNR for me.
Ok, enough bashing. Singing for a rock band is hard to do, very hard to do well, and frankly, it does take a big ego. No one wants to see Mr. Bashful fronting a kick-ass band. Very little pitch reference, and precious few solid cues, singing for a rock band is hard.