I'm not sure if others have said this but... a good difficulty curve is key IMO. You can't have a game be mind-numbingly hard from the getgo, and by the same token your game shouldn't be a breeze right before the final boss/level either. Hope this helps!
I agree 100% that it's crucial for the battles. But, it's at least as important for the puzzles. It sounds obvious, but some game designers forget that not everyone thinks like they do, or knows what they do. The result are puzzles which irritate and confuse players. After all, it's not like players can Google for a walkthrough of your RPG when they're stuck.
But, it really depends on your audience and the RPG. If you are, say, writing a Soccer-centric RPG, it's fair and reasonable to expect good familiarity with the rules of soccer. But, it might not be reasonable to expect that level of familiarity with the rules of soccer for a typical fantasy RPG player. So puzzles which rely on that knowledge will not work for that different audience.
I think a good puzzle is logical, consistent and can be solved using knowledge the audience has, either innately, or given to them in the course of the game. That sounds straightforward, but I've seen commercial games where the walkthrough is required, because the solution requires a strange set of actions, none of which obviously even lead towards the solution.
Just something to keep in mind.
