What makes someone intimidating?

I think the way a girl dresses could amplify or downplay her hotness. For instance, wearing a miniskirt and tube top will draw you all sorts of male attention, whereas dressing in a potato sack won't.

However, I don't think it's the primary reason guys will or won't hit on a girl. For instance, female news anchors on TV dress pretty conservatively, but a lot of guys still consider them to be very hot. I've never felt intimidated by a girl just because of how she dresses (unless she's a soldier). Intimidation probably comes from the guy's subjective assessment of the girl's looks, social status, wealth, and personality (which may or may not be correct). I think this assessment is how we decide who is/isn't out of our league.

I really think it's more location and context. Some places (clubs, bars, etc.) lend themselves to flirting with and approaching girls. Others (stores etc.) may be less appropriate, and the guy won't say/do anything for fear of getting branded a weirdo or stalker. A lot depends on the guy himself, and how comfortable he is approaching women under certain situations.

"if a girl is pretty and wearing a sexy/flattering outfit she will get hit on."

It depends on what kind of people she want to attract to. It is certaintly true that the majority of prostitutes use costumes to lure potential customers. The high-end ones, though, from what I heard, educate themselves a bit more and use more of their brians to attract high-end customers.

"but if a girl is pretty and looks conservative she will be intimidating/unapproachable? is this correct?"

Again, this depends on WHO is looking at her. Those who are looking for prostitutes probably won't find her attractive enough to approach.