Umbran said:
You can have classes of adventure design, and mechanical structure, that you can then mix and match. Go for classification based on structure and intended function, rather than history, and you are unlikely to set off the same alarms. ... so many people have 'fessed up to using so many supposedly "new school" elements back in the 1970s that the historical classification seems outright false ...
Yep, the "old" and "new" may relate chronologically to trends in popularity, but the tendencies of the "schools" have been around since early days.
Rules-heaviness on one hand, and heavy dependence on referee rulings on the other, are preferences best dealt with in choice of rules-set. People with strong preferences gravitate toward what they prefer.
Other preferences have clustered around those, but the associations are not necessarily as they have always been. Some in D&D today seem to me just the reverse of those I see in other reaches of the RPG field. A lot of "play style" choices are about as practically feasible regardless of choice of rules-set (although there are also some practical synergies) -- but some styles may be more popular among players using some sets.
It's in the field of techniques related to those that there's a lot of room for productive discourse. If someone happens to
want to run an old-style ("mega-") dungeon, or an old-style ("sandbox") campaign, then the advice of people with actual experience at doing so may be helpful. Some aspects have been dealt with only obscurely in all but the first rule-books, and a lot of practical lore has been worked out since 1974.
Many "old-school" people are themselves no longer spring chickens. Even in such a "frivolous" pursuit as D&D -- or the blues, or fencing, or animation, or what have you -- there are people interested in handing down old things, and people interested in receiving them both to enjoy and to pass on to the
next generation.
At the same time, "the new school" is no more monolithic than the old. Developments since D&D first fired imaginations have not been on a single track toward "perfection of the form". The initial explosion of game forms has produced not just branches but
threads interweaving in their growth.
As I have recounted elsewhere, when I experimented with a "narrative game system" design back in the '80s, I knew of no other examples. My initial excitement took a cold shower when play-testers averred that the concept would be too hard for many gamers to "get".

When I saw it growing a few years later, it seemed to be significantly an "anti-D&D" phenomenon. Today, it is a well-established part of the D&D scene, not only among hobbyists but at the commercial level shaping development of the "official" game.
I happen to think it best addressed as a new form, as distinct from RPGs as they are from war-games. Because of that lack of personal interest, I have not kept up with efforts rooted in the RPG legacy. That does not mean there's nothing to learn from them, though -- and people versed in such experiments figure among "new school" D&Ders.