You mean like a feedback system? I like the idea but it is never implemented well. My Xbox Live account is 90% avoided because people usually only submit feedback when they have negative experiences. If, however, after you leave a group you are given a prompt to +1/-1 the players you were with it would encourage people to leave positive feedback.
There are so many topics being covered in this thread that it gets confusing. The only thing I see being "exploited" is sitting on a champion so that after a player has deleveled they can kill the same creature over and over. I don't know if this gives more/less XP at a lower AL but I do think a player can delevel and relevel as much as they want, as long as they have to go through the realms again. The problem I seem to remember with that is subleveling gives substantially less XP so something would need to be done about that.