Alot of peoples assumtions of zakk wylde are only based on his recent work, lets say The Blessed Hellride, Mafia and Shot to Hell. Any albums earlier than that have much better lyrical and compositional content (especially P&G and Book of Shadows). Also I've seen him improvise and it can be pretty impressive especially when hes on an acoustic. Zakk actually has a huge range of styles he can play, he's not all pinch harmonics, pentatonic scales and palm mutes. Is he the best out there? not by a long shot, but dont sell him short he's a great player.
Speed is over rated btw :p (I'm sure someone else has said that)