Well, I think there's the OSR, and then there's the retro-clone movement, and then there's the New Wave of Pre-3e Material. I think there's significant overlap between these three groups, but it's worth making a distinction.
I understand the retro-clone movement. It was born in 2006, as the brainchild of Chris "Solomoriah" Gonnerman and Matt "Mythmere" Finch (who as far as I can tell both independently had similar ideas at the same time), who were quite quickly joined by Daniel Proctor and yours truly. BFRPG was first, then OSRIC, then Labyrinth Lord and finally S&W.
The retro-clone movement arises from a fusion of the early-80's rules with the late-90's ideas around Open Gaming. Ryan Dancey is as much its spiritual father as, say, Frank Mentzer is, because retro-clones are basically about empowering
someone else to publish something. And the main retro-clones are free, like the 3e SRD was.
That "free" element was vital, because when the retro-clones came out, the market was wary and cynical. The 2004-2005, d20, OGL-fuelled publishing blitz contained some worthy material and also a lot of overpriced rubbish. The attraction of the retro-clones was that you could see the game you were being offered, download it, read it, think about it and
then decide whether to buy a print copy. Buyers loved that, and so the retro-clone movement got a further boost.
I understand the New Wave of Pre-3e Material. This is what happens when GMs who've been running the same system for 30 years finally get a chance to publish the best stuff from their notes:- publishers have a lot of material to choose from, and they can be a bit selective about what they market. The New Wave will ease in time, but at the moment there's still plenty of pent-up material to sell.
I don't fully understand the OSR. There's an extent to which it seems to be about nerd-rage, self-justification ("our game is better than your game and here's why") and evangelism (on the apparent theory that if you once try a Gygaxian game you'll be "cured" of enjoying later editions). I would obviously like to distance myself from that view, even though I might be accused of having fuelled it by publishing OSRIC... but I
am evangelistic to this extent: whoever you are, I'd like to sit you down at my gaming table to play OSRIC with me.
Negative aspects aside, there's also an extent to which the OSR is a positive and healthy thing, celebrating the rediscovery of all the things Gary Gygax, J. Eric Holmes, Tom Moldvay, Dave Arneson, Len Lakofka et. al. got right. And I can understand that. Extracting a simple and coherent set of rules from the disorganised, rambling stream of consciousness in High Gygaxian prose that is the 1e DMG is no easy task, and it doesn't surprise me in the least that thirty years after that book was originally published, people are still discovering fresh nuggets of wisdom in what it says.