I am seriously considering using Original Dungeons & Dragons, the three Little Beige Books, as the baseline for my new not-quit-named campaign setting. I definitely want a system in or near the direct line of D&D descent, with all the attendant clichés. The other primary contenders are BasicExpert D&D/Labyrinth Lord, 1st edition AD&D/OSRIC, Castles & Crusades/C&C Collector's edition, the Rules Cyclopedia, MERP/RoleMaster, and Holmes Basic/Advanced Holmes. Any of those games would work for what, at its heart, is just another DM's rearrangement of the familiar elements of orcs and swords and such. And I'm not ruling out running session set in the World of [insert name here] with those or any other systems that catch my fancy, like Forward... to Adventure! or Encounter Critical or some Palladium mishmash.
OD&D has my eye right now for several reasons. That's where all this dragoning and dungeoning nonsense began and if I'm going to build an old school sandbox from the ground up, why not start with the tools that my predecessors wielded back in the day? And unlike my beloved Moldvay Basic/Cook Expert combo, I can approach the rules without nostalgia. Seriously, I have trouble finding any flaws in the '81 Basic/Expert rules and that's an unhealthly attitude to have towards a system you actually intend to run. And the wide-open, not-ready-for-prime-time nature of the original books really appeals to me. I can tack stuff on from 1st edition Advanced or some old magazine article or another system entirely and not feel like I'm daring to disturb the universe. In fact, I want to add a bunch of other stuff. I'm not looking to be an OD&D purist.
Another reason why I want to go with OD&D is that chargen is super easy. 3d6 in order. No swapping stat points around. 3d6 x 10 gp and a short equipment list. 3 classes. 4 races. 3 alignments. That's about it. I want players to be able to bring a friend and start playing as quickly as possible. This ties into another reason to go with a lighter incarnation of D&D: for this project I'm not interested in the concept of "rules mastery", whereby dominance in the game is achieved through knowledge and deployment of intricate rules. I want players to succeed by judiciously and imaginatively interacting with the fantasy environment. I don't want the players to look for rules-based solutions, I want them to do cool stuff! For this campaign I'm working within the context that the rules are there primarily for when imagination fails. OD&D supports that approach. All-encompassing editions work against it, in my opinion.