I've been thinking a lot lately about what Mike Mearls calls "core stories," and what Joseph at Greyhawk Grognard has called "campaign tentpoles." Not that they're nearly the same concept, but they both contain the kernel of the same basic concept. Mearls applies it mostly to settings, and Joseph to campaigns -- and a particular kind of campaign at that -- but they both posit that successful game has some kind of repeatable, foundational activity to it, that GMs can vary or diverge from as they please, but that still remains, a touchstone for the campaign no matter how crazy things get.
Although Mearls focuses on the setting elements involved, it's as much mechanical as it is story or setting based, which is why I'm using the term "core activities." D&D's core activities include the "leave civilization, kill things, and take their stuff" elements that he identifies as the core story, but they also include the mechanical tidbits involved in that process -- gaining XP, leveling up, and using new powers and magic items. If you took the core story he identified and ran the game using, say, GURPS, it wouldn't be the same game. While you do gain XP in GURPS, "leveling up" is very different. Such a game could certainly be run, but you'd have to deal with the fact that a GURPS character has much less intrinsic motivation to descend into a dungeon, fight monsters, and find treasure than the average D&D character.
A good core activity makes it much easier to pick up and run a game, and it makes it easier to pick up and run. Pretty much anyone can pick up some D&D books and run a game. Not necessarily a good game, but I know a lot of folks can attest to how much fun even a simple dungeon can be if its new and different from anything they've ever done before. A more experienced DM doesn't need to lean on the core activities as much, if they have some cool new idea they want to try out, but for a long running game its nice to have something fun and familiar for when everyone needs a breather.
Having been inspired by the excellent Storytelling Engines series at Fraggmented, I'm going to take a shot at examining some different core activities, starting with D&D (and its derivitives) and working my way through the rest of the games on my shelf. I'll try to figure out what the major elements of the core activities of each are, what works and what doesn't. In particular, I'm going to try to examine the mechanical aspects of these activities, and how they interact with setting and other social or conventional aspects of play, since that's what got me interested in the topic in the first place.