For a long time i answered this quesiton with "Gameplay". But i don't think thats always true anymore. Games which are story-driven and heavely loaded with story and background information don't need to have a awesome gameplay to be good. So wether it is gameplay or story, depends on the type of game the developer wants to make.
And even just "gameplay" can mean many different things. Is this the open-worldness vs a strict linear progression? Or is it the battle mechanics, level-up and stat-growth? Or does this mean a lot of small things, like minigames or crafting or something completly different? There is no definite anwser, everything depends on the kind of game the developer wants to make.
There is however one aspect which i find most crucial. Not in the sense, that is has to be really great, but if it is done wrong i will quit the game, regardless how awesome the rest ist. That aspect is "Interface design". Now the maker gives us all the basic menus and windows to work with, so in some way it is hard to screw there up. But i have seen my share of bad interface design in rpg2000 and rpg2003 games, where making a custom menu was quite common.
Bad interface design can mean that i need a lot of key presses and go into a lot of sub-menus to do some trivial thing (like use an item or save the game) but it can also mean long fade-ins and fade-outs. Imagine waiting 1-2 seconds for the menu to pop up after you press esc. (Yeah i have seen a game like this)
If the interface is annoying the player spends more time struggling with it than actually playing and experiencing the game. Which will probably in most cases lead to frustration and quitting the game.
As i stated, interface design doesn't have to be really great, but it has to be good. Making a decent interface isn't hard with the rpg-maker software, but it can never hurt to think a little bit about it when you implement some new awesome feature or redesign the look and feel of the game menu.
