Well, in the matter of the right material in the wrong hands...

I once attended a wedding where the singer - who performed fairly decent "lounge" music, i.e. Mack the Knife - embarassed himself and most of the guests by attempting a cover of Proud Mary. It was so ludicrous as to be surreal, kind of like Pat Boone doing heavy metal. (Yes, I know Mr. White Bucks actually put out such a recording and garnered an impressive amount of airplay, too, but that doesn't mean he won't have to answer to a Higher Authority for it someday.)

As for instruments, the tedious overuse of drum machines sometimes gets to me. And groups who all want to sound like Enya (not that I don't, mind you).