Because we have a recent thread about pet peeves that you have when playing RPGs with others, I thought maybe we should start one for people to list some of their more positive experiences or what types of things you have really enjoyed about different groups or people you have played with.
For example, under pet peeves I mentioned a guy who was always in character, even when we were taking a break and eating burgers. Because I didn't know him outside of the game, I actually thought that he was kind of an arrogant jerk because his character was. Since getting to know him outside of the game, I have realized that he is a great guy and a great role player.
So what I would say is that the opposite end of that pet peeve is the fact that I really appreciate the fact that he understood his characters far more deeply than most people I have played with. Sometimes people take a lot of time to assess how their character would respond in a situation. Sometimes people do what they want and pay no attention to the character's personality as they have crafted him. I really appreciate when someone knows their character intimately and has spent enough time with that "person" in their head that their actions in game actual reflect what you would expect from that character. Or even more so, when that character is developed in their own mind enough that their actions surprise you (when you counted on them behaving a certain way), and yet are still fully appropriate to the personality of that character.