Since I've played a lot of different styles of RPGs, I'd like to offer my likes and dislikes of many popular types of games. Hopefully this will add a somewhat fresh viewpoint. (I think this is my longest post yet!!
I think the most important thing in a Role-Playing Game (RPG) is the role the main character(s) play in the outcome of the game. (I just had to state the obvious.)
Now, there are many different styles of RPGs; some stress combat, others stress plot and interactive movies, some stress puzzles, others stress sub-games (Super Mario RPG comes to mind). In short, much of it is subjective and depends on what the user wants to experience/develop.
I personally like many different types of RPGs: Might & Magic, Diablo, Final Fantasy, Secret of Mana, Breath of Fire, Shining Force, etc. They are all good RPGs, but in a very different way.
The only really useful piece of information I might be able to add is the element of weapons/armor/magic availability. Make lots of stuff! I mean really cool things that people can tell/brag to their friends about. Maybe that super-secret ultimate weapon. Or a cool-looking piece of armor. What makes a game really boring to me is when I can't purchase the latest stuff and the monsters are all using it for like 10 battles or so and my characters are getting massacred (think Shining Force II).
But don't ever fall into the trap of making an item just to address a short-coming in the game.
What I am trying to say is to add a lot of variety -- enough so that many different people can play the game and enjoy it, each with slightly different goals. For example, I personally would rather clobber the monsters with a large ax, but that's mostly because the magic in most games is terrible. It's either a real pain to get and not worth the trouble, or really really really limited and simply worse than regular fighting. Diablo was great this way because it let you pick a style of play and then mold your character, equipment, and spells directly to it. And have you ever played it multiplayer using a LAN? WOW! It's like a completely different game!
Very few (if any) RPGs actually show the character's equipped stuff all the time. Why should I buy a Mighty Broadsword of Evil if it looks like an ordinary broadsword? That is very frustrating. And it encourages the player to buy equipment that looks more like the actual character graphic. There must be better ways of keeping character abilities unique throughout the game!!
Finally, what most games overlook is the idea of data security (yes, I know what I'm talking about here). Many games have "cheat" utilities that completely ruin the effect of the game. I say ruin because as a game designer, you want to be in complete control of the user's senses from start to finish, even if it's generated randomly or along several different storylines. That's why games (and gamers, of course) continually seek more realistic graphics, sound, communication, and input. Cheat utilities destroy this control and thus ruin any emotional effects you wish to have upon the player.
Just make it so people with very different backgrounds can play and enjoy the game with different goals and for different reasons and endings. If you try to fit everyone into a stereotype who just likes combat, or plot, or sound, or sub-games, you'll alienate your audience.