Musicians who insist on talking about the piece they're going to play before they play it. 
Special pet peeve:
Musicians who not only insist on talking about the piece they're going to play before they play it, but then say things like, "words can't do justice to the beauty of the piece - I think the only real way to describe it is to play it for you" Duh!
Was at a recital by Jose Franch-Ballester yesterday - he's a talented clarinetist, but he insisted on subjecting us to the most annoying inanities before every piece, and then had the nerve to ask why no one was applauding his 'introductions'. Some people just can't take a hint.
 This is especially true of classical musicians - if you're a jazz artist talking about a piece you've composed that's one thing, but do I really need someone to stand there and tell me how Brahms was a great composer?