ETA: GREAT question - this was really good for me to ponder! And, I totally agree that too much skin showing in the wrong setting can be down right repulsive, and not sexy at all!
I feel sexy when I am:
-wearing clothes that fit well, drape nicely, and are luxurious (well made as well as high-quality material)
-wearing colors I know flatter me
-wearing clothes that are appropriate
I think what it comes down to is that I feel sexy when I'm confident, and I need all these things in order to feel confident.
To answer Maya's question - Yes. I have seen girls in their PJ's and Uggs at Starbucks with their gorgeous hair up in a top knot, looking very sexy. Do I personally like that look? No. I think ppl should wear real clothes in public. Would I personally sport the look? No, and if I were forced to I would feel un-sexy due to requirement 3 above...inappropriate.