This is Inspired by a great article at: http://www.teamfortress.com/post.php?id=2338 and may not be practical for Guild Wars. Just a brainstorm:

The underlying assumption is this: The longer a player stays teamed with another individual, the more likely it is that they are both having a good experience.

There are exceptions to that rule, of course, but in general, it may be true ENOUGH to form the basis of a player rating system. The devil is in the details of course, and the algorithm would have to be well thought-out.

Valve has a system like this (see above link), for server ratings on unofficial servers. The idea is this: The longer a person stays on a game server, the more likely it is that the server is "good". For instance, that server probably didn't falsely advertise number of players or maps played, and it probably has good admins that don't tolerate cheating.

I thought this was a brilliant concept for Valve, and it's reminiscent of Googles elegant "page rank" formulas. (The more sites linking to a web page, the more likely it is that the page is "good").

An analogous system in Guild Wars could work like this:
---- New players start with a score of 0 points
---- Each time a player teams with someone to, they lose 15 points (bear with me)
---- For each minute the player stays teamed together , he earns 1 point (up to some maximum)

The idea is that we want players who quickly abandon others (or get kicked) are penalized, while people who stick around through "thick and thin" are rewarded.

There will always be cases where good players get negative points (beyond their control) just as good Valve servers get negative scores from time to time. This model works on the assumption that good players will get enough points to vastly outweigh those occurences.