D&D's class balance isn't good enough compared to WoW.
Good enough for what?
It's really just a different kind of balance. WoW's focus seems to be on "jack of most trades, master of one" design, where all characters have certain useful abilities, and the classes just distinguish who is the BEST. D&D's balance is more party-based, which means that it does require a party. But hirelings and NPC's have been a part of D&D for a whlie, which means that in an MMO, even if you personally lacked an ability, hiring an NPC with capable AI to perform the task shouldn't be a big deal.
But, again, I'm interested in what you mean by "good enough."
It offers insufficient granularity, so there's both a relatively narrow range of challenges possible (a level 20 monster and a level 1 monster are much closer in hit point and damage potential than a top and bottom level monster in any MMORPG) and relatively few (only 19!) milestones in a character's development to serve as carrots to keep people playing.
What about the milestones of magic items, which serve as 13.3-ish points on the road between levels? And what about "breadth and depth," which includes what many DM's have been doing for years: offering PC's more to do with the levels they have (breadth) other than just gaining levels (depth)? Specifically, in the Planescape MMO idea I posted about, we've got things like planar politics, faction development, and transforming PvP play. In standard D&D/Greyhawk, you still have old classics like building a castle, raising an army, leading the revolt, etc.
In other words, kind of imagine that the "persistant present" of WoW is eclipsed by a gradual advancement of the entire world, based on player actions (players can build castles, found/participate in organizations and governements, etc., in addition to simple level grinding). When a new "patch" came through, organizations and allegiances would shift.
And let's not forget fire-and-forget spells and the relative scarcity of healing compared to other MMORPGs.
But, again, is this a reason why the system would be bad, or just a new way of doing things? "8 hours of rest" can be glossed over pretty simply, as DDO has shown, and scarce healing magic seems more of a setting consideration than an absolute problem to me. Though I could be missing something.
These sound like problems, but I don't yet quite understand why they are problems, per se, and not just "something different."