Why do so many people FAIL at dps.

[Arrius]

Two things:

* There is no way to evaluate your own performance in combat as there are no official DPS meters or any other means of seeing your output.

Yes, and no.
There's the emnity table which you can observe during longer fights. If you see certain DPS being far higher on generating said emnity and start creeping up on the tank, then I assume their output is pretty good in comparison to someone who is even under the healer regarding generation (DPS-healers aside).

However that can be only applied to situations where the gear level is similar and it is a duty which isn't downscaled/synced.