I think there is room for a tank class that (compared to other tanks) does lower damage, but has better defensive stuff. That tank would need agro increase abilities to make up for the loss of DPS.
Knight currently seems like it tries to be such a tank, but fails. Know of any boss that a champ or warden tank can't tank due to needing more defensive stuff? I don't. And the agro abilities that knights do have I've heard repeatedly can't keep up (when I as a ch/r tank am grouped with a knight, they don't tank and don't think they make effort on agro. so I can't comment from personal experience).
So knight seems like a good idea, just poorly implemented at the moment.
That's pretty much it. Champions and Wardens do more damage and thus generate more aggro. The only way to keep them in check with Knights without killing off their potential as DPS classes is to give the Knights a lesser but appropriate boost to their damage. Champions have massive Parry without even trying in addition to being able to swap roles on the fly. Knights have no swap ability to give them the ability to fill another role. The one role they do doesn't really cut it because they have low crit rates, and crits produce double aggro (heard this a long time ago from a friend, so don't quote me).
The primary issue with Knights is that they have plenty of CC skills, but CC is pretty much worthless unless you are running pre-endgame content with an undergeared party. They lack in offensive skills to get aggro and keep aggro by doing damage.
Knight:
-Pro:
--High consistent Defense and parry
--High Utility in party
--CC skills
-Con:
--Low damage
--Low burst aggro
--Low sustained aggro
--No alternate roles in party
You can't kill enemies by having massive amounts of defense and CC alone. "You can't heal him to death..."