I'm pretty new to DD, but right now it seems like the most compelling goal in the Beta is to finish all the quests
, which includes (as a subset) beating all the dungeons, and also requires you to play with a number of different classes in different situations.
If the goal is just to "beat all dungeons", then class balance becomes extremely important, and I don't think there's any way all of the classes will ever be balanced in that way. Some will always be easier to win with than others (generally AND for specific dungeons). And why waste so much effort worrying about class balance when playing with an underpowered or an overpowered class are each fun in their own way? Quests prod you to experience both of these extremes.
(Incidentally, the same balance issue applies to preparation choice. So far I've only seen quests with "no preparations allowed" or "all preparations allowed"... it'd be nice to see some with a preparation limit. Do they exist?)
Whether the goal is "beat all dungeons" or "beat all quests", I guess it's inevitable that you hardcore endgame players will eventually exhaust everything and make up your own challenges. I agree that it'd be a shame if it simply wasn't possible to light up all the "completed with..." classes on the map screen. My OCD nature would rebel.
It'd be nice if there was some way to make it obvious that certain dungeons are only meant for certain classes, without actually preventing endgame players from taking inappropriate classes in and experimenting.