Jeff (aka Captain Liberal) mentioned -- and I agree -- that people in general are friendlier to each other in City of Heroes than I've come to expect from online games in general, and I'm starting to see what makes this game so different from a lot of others.
The fact that there's no player-killing in the game is certainly one aspect of it; the only way you can interact with other people is in a positive way. You can join their team, or you can heal them, or help them fight monsters. But there's more to it than that. The paradigm of this game is different.
In other MMORPG's, like in RPG's in general, the object of the game can best be described as "kill others and take their stuff." I mean, sure, in theory all the things you're killing are evil, but if you think about it that's just your assumption when you play the game. You walk around in the woods, you see an orc. It's just minding its own business, doesn't say a word to you. What do you think to yourself? Hey, I bet that orc has money! Kill it! Once you're outside of town, you can pretty much assume that you want to indiscriminately destroy anything that moves.
From that point, player killing is actually a logical extension of how the game works. I mean, who's going to be richer than another player? Why waste your time killing hundreds of orcs for a few dozen gold a pop, when you can kill one player and earn thousands of gold pieces plus a full set of armor and equipment to boot?
City of Heroes is different in a couple of ways. First of all, the bad guys that you kill are actually doing something wrong to warrant killing. You walk around the city and you see those little word balloons that say things like "Help! I'm being mugged!" Then you run to help the people, and what's your reward? Do they pay you? Do you loot the bodies? No... they THANK YOU, and that's what you get. An ego boost. Or alternatively, if you're playing an indoor mission, you are there because your contact said that there is some kind of illicit activity going on. Again, you're doing it for some kind of societal benefit.
Well sure, you get experience and "influence", which is basically equivalent to money. And sometimes some inspirations or enhancements magically appear in your inventory. But that's the other interesting thing: you get those things automatically. You don't have to scramble to get them before the other players do. There's no competition over limited resources; if you fight for good and cooperate with others, you get rewarded. Period.
In fact, every aspect of this game seems carefully designed to make friendliness not mandatory, but desirable. I can't help thinking that this is a very positive step in the world of RPG design. Your character is not just a wandering cutthroat or a thief. Okay, so he's a vigilante, and that's normally frowned on in polite society. But it's still a step up.