There isn't actually any "Hard Cap" per-say, you still gain EXP from quest turn ins up through Level 12 (I am unsure if 13 is the same), by the time you are 13 the amount of quests you can do has dropped to near zero.
This however seems to be, in my opinion, something that should be changed. Since as far as I'm concerned if level 10 is the Hard Cap then it should be a Hard Cap, no exp (outside DM exp) should be allow at this level. Reason being: the average level of the server seems to sit around 9-11 and it's intent is to have people do fun things with others, but honestly 90% of what I see people doing is Spirmancing. A second contributing factor to this is the penalty from dying, if your PC is level 10 or 11 dying at that level then even being resurrected costs so much EXP this "penalty" starts acting more as a deterrent. Rather then put your PC in a situation where they could die and lose EXP without gaining some direct benefit, most simply seem to either A) do the same one quest, which they know can't kill them, repeatedly or B) Spiremance around.
TLDR: I feel the Hard Cap isn't Hard enough and that there should be more risks taken by people through either quests being far more threatening /or/ there should be some kind of need for PCs to put themselves in more danger.
Obviously: This all changes in v6 since the Hard Cap at 16 is a strict Hard Cap (looking at you Time Stop wizards) and that I have literally no idea how quests will work or how dangerous they are.