Good posts all around.
I agree anything can be abused... and agree gearscore was/is not the answer. (Gearscore only counted the armor/weapon etc score.. not the player's skill etc..)
What I would like to see happen with a rating system may not be possible... and that is to be able to look at a player's worth at a glance. Not sure how to put it in words outside that without stepping on some toes. I usually have a regular group/guild to play with, so its not that big of a deal. However, on occasion our schedules do not work out and I end up having to PUG. I really hate PUGGIN! but only have X amount of time to play on any given evening... so I PUG or do without.
I'd like to be able to tell in glance if the player(s) I'm going to group with stand a chance at clearing some content/dungeon.. whatever, of if they will more than likely waste time - if you know what I mean.
I've given some thought to this before and about the only thing I can come up with is somekind of MVP system for each group/raid etc.. that if a run was successful you could give one vote to the "MVP" - kinda like the system in SWTOR pvp matches. (Not that I agree with how it works mind you).
Maybe something like that would work... although the lack of any MPV votes could also be used to discrimminate... /sigh.