The best compliment I've ever heard given out was this - "You are a student of life." - and as best as I can describe, this means a person wants to learn more about things... things that have nothing to do with their job or their hobbies - this means they ask questions of people to honestly and truly gain knowledge. They are the ones at the party asking with genuine interest - "did you say you collect stamps? that's fascinating - what makes certain stamps valuable over others?"
Since hearing it, I've only given this precious compliment to two people, for the ways they make you feel when they talk with you - like they want to know more because they are learning all then can.
One is Maryl, a friend who luckily enough, married into my family. When I first started dating Neil, she asked me "What do you like about him?"... a question that says so much more about him than 'how did you meet?' or 'what does he do?' and it was so clear to me that she wanted to know about Neil.
The other is Ira Glass, host of the radio show "This American Life". And I'm slightly crazy for having said this about Ira because I never met him - but I feel like I know him because he is so knowable, and he chuckles, and he says things like "so then what in the world could you have been thinking?!?" and the person he's speaking to just opens up.
Lately, I try hard to ask probing questions that get to deeper answers that tell me things about the world. I say "try hard" because while I love to know about life, having a terrible headache makes it hard to stay engaged. But my hope is that, in time, I'll can become a little more Glass-y.