They need to have a vision. A vision of the world you want to create. In the days of the NES and SNES you could get away with a bird's eye view of the overworld and get away with it, giving that illusion of grand scale. You can't do that anymore. All games are linear at their core but it is important that a game maintains an illusion to keep that person hooked on it. Something that will keep people coming back to play it over and over again.
A vision can entail many things.
It can be a well written plotline with well developed and likable, maybe even characters you may relate to, but the goal is to make them what we are. Human. The plot should also establish the rules of the world you want to create and abide by those rules. And during the progression of the plotline is the character static? Are they dynamically changing as you progress? Do these changes make sense? Do they not? All these must be considered. Regardless if the plot is generic and simply good guy beats bad guy or if a story is deep and invokes deeper thought via game changing choices that diverge story paths.
Graphical fidelity is important but only in regards to what you want your world to look like. Good graphics, animation and whatnot is needed for that visual appeal. But is not always required depending on your vision. How does the world react to your actions, how do the characters animate. Quite important. It doesn't need to be the best out there but should take advantage of the full power of the system it is running on.
Gameplay is one of the most important aspects. Is it fun? Is there room for improvement? Is there a simple way to use a specific gameplay system or is there more challenging aspects of the gameplay system? Is it balanced? These must be considered. Not to mention different RPG's may have a variety of different gameplay aspects. Battling an enemy. Competing in a card game of sorts. A racing minigame. Vehicle travel. And much more.
The most important aspect is always one thing. Execution.
If you cannot meld Story, Graphics, and Gameplay into one it is often it will not hold it's audience's gaze for long, and may not have them return to experience this vision over and over again. If the story is asinine, if the graphics are poor and hard to look at, if the gameplay is stale or simply not fun then I think while you may have some good idea's they may not mesh well and thus fail.
That is what I think needs to be considered for making a good RPG.
However I am a mere gamer so my words can hold meaning or may not hold any meaning at all.