what do top- and mid-tier sororities judge girls o

How much do looks factor into getting a bid? Prior achievements? Personality? Family background? Intelligence?
What's the number-one reason why a girl wouldn't get a bid, if we specify "she wasn't a good match for us"? (Related: what's a dealbreaker?)
I'm planning to rush in the spring (which I've heard is incredibly competitive) and would like to know if it's even worth bothering visiting top- and mid-tier houses that'll be participating in spring recruitment.