All editions of D&D were, are, and forever will be "playable." The problem, though, is that up until 3E, D&D was a kind of Frankenstein's monster - many disparate pieces sewn together. 3E tore the whole thing down and built it back up from the ground with a stronger core foundation, one akin to other third wave* RPGs which were designed with core mechanics and tidier rules.
This is why I don't think any OSR game will ever become a big force again - they're all just too anachronistic, too cobbled together. Game design has moved on. The closest thing to an OSR game that, I think, has the design integrity to be successful long-term is Castles & Crusades, which is sort of like "3E Lite" but going for an OSR vibe. But I don't think it has the publishing pizazz or marketing chutzpah to do it. Plus the name implies medieval simulation, not fantasy heroism. They would have been better off with something more fantastical.
*By "third wave" I mean the RPGs like Talislanta and Ars Magica, and then Vampire and Mage, that started coming out in the late 80s and early 90s that had tighter mechanics. First wave with be pretty much any version of D&D before 3E; second wave would be games like Runequest and Rolemaster that were doing something similar to D&D, but in a different way. In this sense, 3E was a third wave game, even though it came out around the time of the "Indie Revolution" and would could be called fourth wave games. But I'm just making this up as I go along...