As far as I can see, there is one glaringly obvious problem with a D&D movie: D&D doesn't have a story. The books are really just a set of rules for how to come up with a story of your own. Really, the only ways I can think of to make a D&D movie, or at least a goodish one, would be to either just make up a fantasy story and sort of say "This is D&D. We promise!" or to make it more about D&D in the way that The Social Network was about Facebook than the way that Halo novels (I hope there's never ever ever going to be a movie made about those, and if there already is one, I'm going to pretend there isn't) are about Halo.
Anyway, the thing about D&D is that it doesn't have one specific story. Yes, it has a whole fleshed-out mythology, but it's really meant to be more of a jumping-off point for a Dungeon Master to write his own story. Theoretically, you could use the d20 system, but not use any races, classes, skills, feats, spells, etc. that Wizards of the Coast provides. It's really more of a way of telling a story than a story. To be honest, it would actually make sense to see a WoW movie before a D&D movie, because WoW actually has a specific, singular plot (or so I've been assured by people who play a lot more than me).
So anyway, yeah, thanks for reading this far. As a special treat for you for not getting bored by my odd internal monologue that somehow made it through my fingers to the internet via a keyboard, I say Obey the Wombat Overlords. <angry yet non-threatening fist-shake>OBEY!</angry yet non-threatening fist-shake> Ohandbythewaythiswebsiteisawesomeyaykthxbai.