The worst example of defanging came with the uber vamps. They seem to be like Daleks. One on it's own is the most terrifying opponant imaginable, one who Buffy only just takes down (with a lot of help as I recall) but an army of ubervamps seem to be made of tissue paper...
That's down to what TV Tropes calls the law of conservation of ninjitsu...
http://tvtropes.org/pmwiki/pmwiki.php/Main/ConservationOfNinjutsu