Ah, RPGs, that most nebulous of genres.
Personally, I don't like the online scene, too much humanity of today taints the immersion and also, I despise player killers yet I do see their point (*urk* much as it pains me to say it). Anyway, it has nothing to do with whether a game is online or not, but rather the linearity of the game.
So here I go on my rant which is merely that, a rant. Ignore at your own leisure.
You have the Final Fantasy's, in which plot and character development is paramount. Highly linear.
Then you have the RPG high games in which there is a plot, but you can follow it at your leisure (which also means, the world revolves around you - but much can be sad about Final Fantasy). Highly interactive. Here we have Darklands and Arena.
Then we have the many shades of grey in between where one can find the Fallouts and what not. Of course, I ignore the RPG-lite games like Diablo (merely RPGs for FPS fans in my opinion *ducks flaming ball*).
Ok, on to the actual question far above...
What makes it a hit? Thoroughly unknown. Look at Betrayl at Krondor. When it came out back in the days of diskette, it did not do well. When it was rereleased on CD, Boom!
It's a market thing. Just pray, even if you did make a good game. As long as you are in the black, everything's good!
Plot: Don't cliche it is the main thing. If you must use a cliche, use it, but then twist it.
Graphics: People are spoilt now. 3D or 2D? 3D requires machine (Ultima 9 anyone?). 2D requires a lot of art (Baldur's Gate anyone). 2D/3D hybrid is my opinion the best to go. Then again, if the game's good, 30 hours into the game and graphics don't mean anything. Gameplay keeps the player.
Music: I'm partial to classical, but if you mix classical training into anything else, you've got something. But music is highly subjective.
Action: Ah, lots of it? Be careful. Most of this has been covered by others.
Hmmm, I haven't really answered what makes a RPG a hit. It depends on the person. I don't like (and boy, I've tried) 3D first person perspective games. I like it even less when combat is required to be done manually, as in point and shoot (Eye of the Beholder although first person had simple combat - point and click). I prefer the abstract simplicity of the strategy in Fallout and the Gold Box SSI games, or the simple menu select method of Final Fantasy, or the point and click method of Baldur's Gate. But that's just me. As long as music is not too repetitive, it's ok, unless it's something I like to which case, I don't care if it is. I'll pause the game and listen. But I feel I'm in the minority mainly because of this 3D thing (and I do occasionally play FPS, they just don't hold my attention for more that 10 hours cumulative play, even in multiplayer).
Just my gaming habits and I feel more a casual gamer rather than a hardcore gamer. Now the disclaimer: The opinions of this poster does not necessarily reflect the opinions of the gaming community.