I think it would be good if you could rate players i.e 75%good Tomatoes 25% bad or by stars 1-5 like you do in like ebay with buyers and sellers and this could be on your profile
You could make it so you could rate a player only after you've played with/against them, and add some sort of minimum level etc. One of the most interesting things about communities is how well they do when there is some sort of karma system even though like Richard pointed out, trolls can use it to their advantage. I've managed several communities though and have seen more positive things happen than negative.