Some of what I've seen and/or experienced:
Blazer or suit jacket's shoulders are too wide or narrow - the latter hurts while the former makes you look slightly robotic or as if you are wearing shoulder pads.
Gaudy t-shirts - man, you must have a lot of confidence and ego to even attempt wearing a loud and gaudy shirt. You probably also don't mind blinding people with your Technicolor™ Dreamcoat, either.
Too many shirt buttons unbuttoned - you might as well unbutton it all the way or forego the shirt in the first place.
Pants halfway down the derrière - unless you have a mic in your hand and you're rapping about your money/female troubles against a booming backbeat and your name is Jay-Z, this should not be seen.
Pants that are too big or small, too long or short - a huge problem. Too big and you look like you're wearing a skirt. Too small and it looks like you might be in pain and having a semi-erection all the time. Too long and you look short child who accidentally wore his father's pants. Too short and it looks like you can't wash your clothes properly or you're wearing capri pants - I have yet to see a man be able to pull off wearing capris.
Pants without belts - just plain look bizarre.
Butt cleavage - leave that to your plumbing professional.
No underwear or wearing thongs - shall we keep shaking hands and touching things in general to a minimum?
Most of your underwear shows when bent down/over - not a big fan. It reminds me too much of fat guys and strip shows, which conflates to fat guys who do strip shows - not a pleasant image. Also, the situation seems so unfair that I feel compelled to show you my underwear in exchange and no one wants that.
Too much cleavage - showing some is fine, but excessive cleavage is not so good if you want a man to talk to and look into your eyes and not your breasts.
*Revealing skin in general - to me, there is a lot of gray area here. Some women can wear a lot of clothing and still look fantastic. It depends on your comfort, confidence, body type, the type of look you are going for, and the kind of attention you want to attract. Personally, it seems more skill and fashion sense are involved in looking more attractive with more clothing than with less.