I've gotten a little encouragement to post more often, not sure I'll actually do so but I do have a couple of "meaty" topics in my head that I'd like to post about either here or on a forum. So these are just notes toward more expansive treatments.
1. Exogenous vs. endogenous fun in mechanics
Noting that what everyone nowadays sees as the "core" of D&D's mechanics, i.e., the hit tables and hit dice, was originally just something slotted in to replace Chainmail. And Chainmail was also just grabbed off the shelf by Arneson to fill a fairly small role in his Blackmoor game--the point of impact in combat. The full game (including OD&D) was actually one of exploration, puzzle solving, characterization, spells, mysteries, etc. blah blah blah. The combat rules by themselves were really pretty pathetic, not a very fun game, but they filled a necessary role and yielded output that enriched the game as a whole. This is exogenous fun.
Compare the combat mechanics in The Fantasy Trip, or (very likely) D&D 3.x and up: here the combat rules are inherently fun, a minigame. In fact TFT started as a couple of arena combat games. In games that swing this way, the module/campaign is in a way an adjunct to the central mechanic--it provides an excuse or framing mechanism for generating scenarios or skirmishes or what-have-you, and enhances it by adding some stakes to the outcomes. This is endogenous fun.
It may be worth constructing a third category, essentially encompassing the pure stakes+narration-trading style of many new games, where the core mechanic is neither inherently fun nor particularly "productive"--it doesn't by itself generate stuff to play around with, instead it just arbitrates between options proposed by the players. Personally I don't care for this style but it seems to exist, apparently has its fans, and I believe it's worth distinguishing from the other two. But perhaps you can see that I'm struggling to describe exactly how, and what it does.
2. Getting beyond System Doesn't Matter and System Does Matter
Really, the whole thing is silly, these are presented by the naive in maximalist fashion, and it's a mistake to take them seriously. What should be done is to examine a little more how system matters; this is related to point 1 above, and is nicely shown up by arguments over what people are concerned about with D&D 4e. There are issues both of formal mechanics, dress (i.e., hype), and informal guidelines. There's also a historical dimension here as it seems with many games there's been a natural progression from system mastery to transcending system--and I wonder why that can't happen with 4e.
3. Finally, why houseruling isn't the same as creating a new game--the point I made which generated the abovementioned encouragement. This is really a return to Chris Lehrich's talk about bricolage but it may need to be said again in a new way with different emphases.