1. Exploration based gaming as a supported mode of play, where the players have to weigh diminishing resources (spells, hit points, torches, food, ammunition, etc.) and increased risks (the farther you go from home base, the badder the monsters are) against increased rewards (more treasure the farther you are from home base) as they proceed in the adventure.
2. A true 1e style Vancian magic option. By this I mean, the low-level one-shot cannon, not the 4 shot pop gun of 3e. An increased emphasis on magic with long-term effects from early levels (Charm Person in OD&D and 1e could last months or longer, for example), especially non-combat effects, rather than flash-bang type of magic.
3. Quickly resolved, grid-less combat option.
4. Some sort of objective xp award system option that isn't killing monsters. It doesn't have to be gp=xp (although, I still think that's the best way to go), but something that is neither "kill every monster you meet" nor "jump through the DM's story hoops" while still being relatively easy to track.
5. Goes hand-in-hand with #1... Monster stat blocks have to be reasonable enough to allow for quickly generated wandering monster encounters and very large dungeons and wilderness areas to explore.
6. Already mentioned by others... bring the npc back into the party. I want rules for henchmen, hirelings, cronies, followers, specialists, and other various hangers-on. This is what charisma was for originally. (Look it up. The attribute the gets easily the most ink in OD&D is charisma and it was for this reason.) The logical conclusion of the "building your support staff" mini-game should be that the pc ends up with some political power of some sort, and there should be some DM guidance (minimal as in 1e or incredibly detailed as in the C portion of BECM) in this regard.
7. Related to my #6 and also mentioned by other posters... There needs to be a morale mechanic for npcs and monsters. B/X D&D did it the best. Assign npcs/monsters a morale score between 2 and 12, roll 2d12 under certain situations, roll the number or under morale holds, roll over and morale breaks. Simple, elegant, works great.