If you look at some of the original RPG's their stories were no more complex then the original side scrollers though. We may come to expect that these days, but it's not always the case.

In terms of system, I guess I would define an RPG by, in terms of combats, being heavily a numbers game.

You get games like the original FF where there is no twitch reflexes involved, just planning your attacks based off the damage they shall do or mitigate.

Compare that any FPS, where your focus is a lot more on reactions, timing, ect. Compared to an RPG FPS's aren't near the numbers game.

Yes, pretty much all games are a numbers game, but an RPG is a genre that takes that to one of it's logical extremes.