First, keep the base system simple, lightweight and intuitive.
D&D is the flagship game of the entire hobby, and will most likely be the first rpg experience that many potential gamers have.
Furthermore, the vast majority of players aren't here on the forums...they're out doing whatever it is they do when it's not game night. Most barely even bother to look at a rulebook when they're not at the table.
Newer and more casual players should be at the forefront of the designers minds when they are finalizing details of the new game. Avoid writing deliberately obtuse mechanics and needlessly complex, esoteric rules when a simpler system will do.
There are plenty of other games out there designed to be an impenetrable dedicated hobbyist stronghold...D&D shouldn't be that game.
DO make the game fun and easy to run.
If one of the goals for D&D Next is to attract new players, then one of the best way to do that is to create more DM's. In that regard, the game should be as DM friendly as possible. An easy, user-friendly and intuitive system allows DM's to make quick, fair and consistent rulings on the fly, without cluttering up the page space with rules for every single corner case scenario.