I've now completed all the current class challenges except for Fighter and Bloodmage gold. They all seem reasonably well balanced with the caveats I've mentioned here and elsewhere (Thief Gold dungeon generation, etc.). Berserker Gold, though, is by far the easiest of the gold scenarios, and IMO could stand to have its difficulty increased a bit. Once you figure out the blatantly obvious gimmick, the boss becomes a pushover. (I beat it on my first serious attempt and got Hoarder - would have gotten Feeling Parched, too, except I used my health potions early on to level up since they'd be useless against the boss.)
Namtar's Lair and the Library could probably both use a quest - even if the reward is just gold, they are (or can be) huge gold sinks with almost no gold reward, so giving something back would be nice for those players who have just spent a small fortune on preps. Or maybe the item trophy dungeons could just have normal trophies, too? Or, perhaps, secret gold vaults that you can enter after you beat the boss? (That would make quite a bit of sense with the Matron, given the message you get about her subdungeon before it's unlocked.)