It could be a matter of slang... like the way "bad" in standard meaning means "poor quality, unfortunate, unpleasant, etc." but in slang it can mean "cool, awesome, hip, etc." It could also be a matter of language drift as well.
At any rate, every definition I've personally come across gives "my" definition for "kvetch", and I've heard some of my Jewish friends use it that way, too. Still, Bushie's definition isn't too far off logically... it's not a big leap from "lots of loud griping" to "crap" (or vice versa), if you think about it.

Storm's definition is a bit harder to follow back and forth.
Peace & Luv, Liz