You can learn lots, Phil Owl. One thing you can tell, as a girl, is how bossy they are...their lead can tell so much about their attitude. A strong lead generally means they're confident, a pushy lead usually is the result of them being pushy and a weak lead usually means they aren't a confident dancer or that they're a little weak in their own mind. They're fairly large generalisations, but I think a number of women might agree.
It's funny how sometimes you can tell who's agressive and who's not by how they dance with you. There are some people who are unfailingly graceful and kind, no matter how bad you are and others who make it patently obvious that you're a dud. Don't ever expect them to dance with you again.
Having said all that while a lead can speak volumes about a person it's by no means the only way to piece together a picture of their personality, particularly if you've only ever danced with them in the short bursts of progressive dances.
One man I gotten to know recently sits, placid and observant at socials. He's quiet, doesn't mix and, from outward appearance, is a sour puss who never smiles. Having gotten to know him I've discovered he is one of the best dancers at my studio. He's also funny and charming to boot. This, despite his sour demeanor and exterior. He also has a strong, elegant and purposeful lead and seems to be the same way in life.