I think the biggest challenges would be:
1. not seeming like a LotR/Hobbit rip-off (in a lot of ways, we already have hugely successful D&D movies right there... however unfaithful they might seem to some)
2. creating something unique enough and broad enough to appeal to the greatest possible audience
3. not coming off as cheesy and a cliché (oh, look, ANOTHER dragon movie... *yawn*, oh, and another ax-wielding dwarf speaking in Scottish accent... *sigh*)
4. developing the characters and plot to a meaningful level in less than 3-4 hours
5. making something that will promote the D&D franchise, possibly expanding it, and not hurt it
I think a simpler, not quite-so-epic, story with a deeper plot that can show the rest of the world D&D isn't just about fighting stuff on adventures.
Now, if they plan to make a 3-movie franchise (also sort of "expected" nowadays... another *yawn*), then they could make something larger in scope I suppose. But I agree with some others, that a mini-series would better serve the game than a single movie (or even three).