If the game isn't going to permit players to evaluate themselves or each other, it needs to do so itself. Someone mentioned WOW's Proving Grounds earlier, and from the abjectly terrible state of some of the DPS I've encountered in dungeons (and also tanks and healers, nobody is immune from ineptitude), something similar is definitely required here.

The funny thing about WOW's proving grounds were that the forums were usually split more or less equally between people complaining that it was absurdly trivial and completely useless as a barrier to entry, and people complaining that it was ridiculously difficult and a completely unfair barrier to endgame content. None of them required anything much more than basic knowledge of your role and how to deal with simple mechanics, but it was still too much for a lot of people, and that's what we see in this game time and again.

I know it sounds horrible and elitist, but there comes a certain point where people can't complete stuff while adequately geared for the content in hand, and the only real answer to that is "get good". But without the tools to properly measure performance, or any effective roadblocks to prevent people from getting carried through content by other players, nothing changes.