Since there's a debate going on about "old school" again, here are some links to essays written so far.
Grognardia: More Than a Feeling
Wondrous Imaginations: My Response to the Grognardia essay "More Than a Feeling"
LotFP: RPG: What is Old School?
And people are asking, why does it matter? Why is it important that you're able to draw some line between "old school" and "new school" in gaming if everyone has fun?
The answer isn't that hard to find, and I think it falls in two places. One is related to the gaming community, and the other is related to products that are being released.
With the gaming community, what we're really doing is pushing back against decades of "new=better" and stating an active preference for play styles that are derived in part or in whole from the early days of the hobby. There are fairly specific things that are involved here: the twin concepts of the megadungeon and the hexcrawl as sandboxes; the idea that sometimes less is more, specifically in regard to "how much do you need in terms of rules depth?"; the idea of player skill lying more in the exploration part of the game rather than in working character generation or the combat system. There are more, none of which encapsulates "old school" but a group of which, taken together, push you over the border line into "old school" territory. A lot of people nowadays seem to have a hypersensitivity to polemical speech, where you basically go further down one direction because you badly need to correct from the other way; this is where we get the whole "rulings not rules" discussion from. It's not that "x is universally bad," it's that we need much more y and less x, so we're making the full-court press argument against x.
To this end, I think we've been somewhat successful. Some of these old school ideas have taken on a broader currency, and people are now legitimately interested in the whole sandbox idea coming from totally non-old school traditions of play. What's being missed is that they aren't playing old school games, just drawing ideas from the old school movement. If you're playing 4th edition with some ideas that you got from people who consciously play in older styles, and you enjoy doing so, that's great. But you're not playing an old school game, if the phrase is to have any meaning. There are big assumptions in between you and old school. This is only a problem because there's a certain cachet that now comes with the whole "old school" label, and people are trying to water it down to the point where it no longer matters.
And that brings me into the other side of why old school is important. Simply put, old school is a quality filter. Blogs, message boards, and commercial releases are still relatively well sorted into "old school" and "new school." Given the limited resources (time / money) I have for gaming, and the fact that my tastes run decidedly on the old school side, I am able to use the "old school" designation as a limit for where I will invest my resources. You can argue that I'm unfairly excluding "new school" material that I might enjoy, which is absolutely true, but I don't consider it worth my time to research and buy new school products in hope that some of them will have been worth checking out.
It's important for me, then, that "old school" stays in tact as a label that filters relatively well along the lines that it has so far. I haven't found every single old school module I've bought to be a revelation, but I've generally found them to be reasonably well written dungeons without an overbearing plotline, which is nice. The problem is that, as people find that there's a market for "old school," there is some necessary dilution of the label as something worth using to differentiate my stuff from other people's stuff. The more we can push back against that, say "it's a nice product but it's x where old school stuff tends to be y," the more we can preserve the old school label as a firewall. It'll never be foolproof, but it's good enough for my purposes now and I'd like to see it stay that way.
Really, it's the same thing for play. If I say "I want to run an old school game," that means I expect people to not mind that they will be rolling 3d6, quite possibly in order, for stats that aren't all-important and all-determining; that I will frequently be making judgment calls on what they are doing rather than referring to a rulebook; that we are playing a dungeon crawl and PCs are liable to die at any time if they're not careful (and it's never careful to engage in combat). Oh, and if you do die, your next PC will probably be in the next room with monsters, bound and gagged, and a first level shmuck. If you're lucky and careful, this shmuck may actually advance up to being a hero, but he or she certainly doesn't start off as one. If that kills the game for you, honestly, why would you want the old school label in the first place? It isn't useful for either side – the people who don't have old school gaming values don't enjoy it, and the people who do want to use it to find players who actually like the kind of games they play.
Everybody (except for Jim Raggi) is in this to have fun. But part of being mature means that you recognize that one size doesn't fit all. My fun isn't necessarily your fun, and these divisons exist for a reason. We're trying to get games, modules, supplements and discussions going for the kind of things we like here. And it's working, which is why I think the "old school is just a feeling" thing is actively harmful at this point.