Joey's tendancy to do those cheesy power metal screams on the earlier albums annoyed me. I actually liked his performance on Persistance of Time, but his live work from the Killer A's video annoyed me. He was constantly going off-key, getting out-of-breath, and acting like a spaz. That's a bad combination for a metal singer, and I hear it's a big part of why he was fired.
There's that, and rumors of "personal problems". I saw someone in another thread make a vague reference to alcoholism issues. I don't know if Joey was really native american or not (he's wearing the "injun" cap as far back as the cartoons in STD, and the native community has a history of problems with drinking, substance abuse, and depression), but Anthrax doesn't need issues like that. They have enough shit to deal with as it is.
I don't know if Joey's cleaned up anything that might have been fucking up his life with Anthrax, but I wish him the best regardless. Right now, John Bush is doing an excellent job, and if it ain't broke, ya don't fix it. I know that a lot of people accuse him of being a "generic metal singer", but he has a sense of lyrical rhythm and melody that Joey could never touch. THe man could read a wine list, and it'd sound cool. Joey was fun, but eternally frustrating. John works. I go with John.