-Self-Sustaining: Players that always make characters with their own motivations; if I don't have anything specific for them to do, they will go off and do their own thing. Also, it's great when they don't need an NPC to have a conversation. There's nothing I like more being able to sit back and watch my players break into a 15 minute constructive dialogue with eachother IC.
-Non-Disruptive: These would be players that don't drift OOC too often, and if they do, it doesn't stop the game or disrupt the mood. I admit, I'm guilty of slipping OOC from time to time, but I don't blurt out a stupid pun when a character has learned that his entire family has died.
If you're not actively involved in a scene, I can't expect you to sit there and do nothing all the time, but if you need to say something to the player sitting right next to you, whisper. Or if you need to say something to one player this absolute second, and they're across the table, get up and walk over to them. Alternatively, write it down, especially if it's funny. Then it can be shared after the game, and much joy shall be had by all!
-Awarenss: This isn't in the sense that I expect my players to absorb every tiny detail I throw at them. Instead, being able to recognize a situation and act appropriately. I have no problem with players stealing the spot-light from time to time, in fact it makes for some of the most memorable sessions ever, but if you notice that you're the only one doing it, and everyone else is sitting by and watching, try and get them involved.
Also, I love a player that can stay in character all the time, but I appreciate it so much more when they notice a situation where it would be better to do something slightly out of character. There have been times where players have had their characters make decisions that will knowingly derail the entire campaign simply because "My character wouldn't do that." (the Paladin syndrome). Unless it is something that is the absolute polar opposite of what your character stands for, if you know that the GM/ST needs you to do it to further the game, then, like the Nike ad says, Just Do It.