I don't think a reputation system would work at all, unless you had real human beings auditing the logs of every dungeon and making a decision about whether negative rep was justified. Is it okay to roll Need for an Offspec when people Need an item for their main spec? Some would say yes, some would say no. Perhaps a more complex version of "Need Before Greed" is necessary, and you'd force people to decide their priorities before entering a Dungeon....but even that would be too complex.
The deeper problem is that there isn't an opportunity to be an asshole that often. How many times do you really get the chance to ninja something out from underneath the rug of a tank or healer? Maybe 1 out of 10. So ultimately you might get a negative feedback every now and again if you're an asshole, but because you're not an asshole that often you still wind up with 90%+ reputation. It wouldn't be because you're actually a considerate player, it'd just be because you don't get the chance to be a jerk most of the time.
The deeper problem is that there isn't an opportunity to be an asshole that often. How many times do you really get the chance to ninja something out from underneath the rug of a tank or healer? Maybe 1 out of 10. So ultimately you might get a negative feedback every now and again if you're an asshole, but because you're not an asshole that often you still wind up with 90%+ reputation. It wouldn't be because you're actually a considerate player, it'd just be because you don't get the chance to be a jerk most of the time.