We're averaging the ratings of team members, but then bumping up the team's rating to account for the difficulty a single player has taking on large teams. Right now we're going with 200 points per team member; that may be too much, it may not be enough. However, I think it's fair to say that if you're on the smaller team, you've got to be significantly better than your opponents (on average) in order to beat a larger team. This change to ratings is used purely for matchmaking, because it represents a prediction of your ability to win not related to your actual skill.
We aren't scaling the size of your rating change based on how far you are from the aggregate score. This is partially because of the number of contortions the final score is undergoing; it's also because we haven't yet done the math to insure that the average number of points in the system stays the same.
We aren't scaling the amount of rating change based on how new the competitors are. The main benefits of this require that the average rating across all players fluctuates, which we've ultimately decided to avoid.
We are taking differing levels and power levels (essentially, how close to the maximum power for your level your build is) into account. This is a bonus to your rating (again, purely for match-making purposes) of between 0 and 200 points for level, so there is a gulf of 198 rating points between a level 1 character in PvP and a level 100 character of equal skill. The bonus based on power level is between 0 and 400 points, both on linear scales.
This all boils down to trying to make the PvP matches you get into as fair as possible (even taking into account information available about characters new to PvP, which helps give new PvPers a better experience), while still keeping the wide open feeling of our current PvP system. Anyone can be matched with anyone... if you're good enough, or have enough other odds in your favor. As a second option, though, if we can't find you a fair fight, we'll find you some kind of fight.
Finally, this means that - although we can have level brackets and team size brackets and power brackets - individual ratings should be roughly comparable across the range of levels, team sizes, and powers. Hopefully we can get this data out to players as soon as it is visible in game!
My PC at home is in a weird state as regards 3D acceleration, and it has me kind of bummed out. It looks kind of like a driver issue (updating to the latest ATI drivers certainly improved things), but I also kind of suspect my video card is trying to die. So far, every game I've tested has some kind of issue: Dungeon Runners crashes, Dawn of War crashes some of the time and refuses to play in full screen mode the rest of the time, and so on (City of Villains looks particularly bad).
