More and more, I treasure a diverse group of players. A table full of white dudes is a missed opportunity for a better game, I think. More women. More kids. More people from backgrounds I don't share, bringing elements to the table that otherwise would never be there.
Also? Laughter. I don't want to screw around, but I want to have fun, and while tension can be fun, I want to see smiles, even if they're caused by pained chuckles from bad die rolls.
Finally: engagement. Throw in your ideas. Get excited and start making stuff up. That's my favorite thing.
Edit to add: Like, I play mostly over Hangouts or Roll20 these days, so I would be remiss if I did not mention how much good, reliable tech contributes to a good session. Solid connections, good sound, considerate microphone use, et cetera.