I've just been revisiting some old articles on RPGs, and it seems that a lot of people are of the opinion that an RPG contains two basic elements that you mix. A clear example of this thinking can be found here, but it boils down to the idea that an RPG is about leveling or progressing the story. Those are the two "modes" of play. All the other rules are to flesh out the modes.
I sort of agree, but it's missing something important. See, to me, the two modes should be called "exploring the options" and "adding new options".
The part of an RPG where you are leveling, buying gear, figuring out good tactics, and so on... that's you exploring the gameplay terrain. The part of the RPG where you go to the next town, fight the boss, and/or get captured is you adding new gameplay terrain.
Some players like to explore every nook before moving even one step down the story, while others like to play it faster and looser, only exploring enough to get secure footing before looking for the next horizon. A well-designed RPG allows both kinds of player to have fun, although most are tilted one way or another.
In case it's not absolutely clear what I'm talking about, think of any classic RPG. You can run around and level up, buy the cool gear, get the new spells, maybe do some side quests for some optional stuff. However, you will eventually run out of room. The enemies don't give you enough XP to keep leveling indefinitely, there's no better weapon than the one you have, and so on. So you fight the big monster, and a new city is unlocked. More powerful monsters, a new weapon, and so on.
Tactical RPGs generally have a much stronger bent towards letting you explore the rules without advancing the plot. In Final Fantasy Tactics and the like, you can level up basically indefinitely using random encounters. Still, you will find it's very hard if not impossible to get the high-level gear and/or spells without actually advancing the plot.
What this boils down to is that an RPG reveals some small part of its gameplay. It lets you explore that all you want, and then it lets you choose to staple on some more gameplay. Which it lets you explore.
This works best when the new gameplay is similar but distinct. If it's too similar, it doesn't feel different enough to bother exploring. If it's too distinct, it feels like you wasted all the time you spent on the earlier gameplay. Just this particular aspect could be a whole essay.
Anyway, that's how I think of it: letting you explore, then letting you open more stuff to explore.
It would be interesting to see if that could be done in a tabletop RPG. Hm.