This topic seems to pop up at least once a year, shame they don't sticky it or something (along with topics about original characters).
If done right/with thought put into it, it can be fun. I agree with Cart-RAT on sexy vs slutty. It's just a matter of being tasteful. I'm not a huge fan of sticking a character in a skimpy outfit when that character's personality doesn't seem like he would wear something like that, if he were a girl. Won't hold it against anyone who does though. Just like with any other cosplay you don't particularly like, just look away.
Eir Stegalkin (GW2)
Alita (Murder Princess)
Black Knight (WoW) -?
Nene (Bubblegum Crisis) -?