Well, first one has to look at the really good games. For me, Baldurs Gate II shines as the best RPG ever made, but thats ofcourse just my observation. If I am to enjoy an RPG, it needs a great story which is masterfuly told, it needs villains you can believe, ie: not cackling, skullmask wearing necromancers, but villains with motivations you can understand, and also a faint glimmer of "good" so they can be precieved as 'human' aswell. If you from some twisted point of view can understand the villain, then you ahve made a good villain.
Second, you need good companions with depth to their character, believable motivations and fears etc. Baldurs Gate I & II did that very well. Quests for your companions is also very nice, aslong as they are not overdone, and take the focus away from the main story.
Third, i want dilemmas. This is where i enjoyed the Witcher; the choices you had to make, and didnt get to see the result of the choices before much later. It adds alot of excitement to the gameplay when you cannot allways predict the consequences of your actions
Fourth; a somewhat original story. The Evil which has been released and must once again be sealed for the tenth or so time is only fun for so many games. How about making a game when there is no moral at the end? And with, as others have pointed out, more than just a good or evil ending. And what of your companions? What of the world? If you play as a mercenarie, perhaps you wont give a damn about that town you encountered way back in the game, but saves another. Or where you have to choose who gets to live and who must die? Or play a thief. Or make an assassin RPG where you dont have to be a nice or totally badass assassin, but things in between aswell. And for the love of God, a game where you -dont- have to save the whole world? How about just a city? Or just your own guild in the city?
Fifth; Put some clothes on those female NPC's. Please. If i want to see hot, lightly dressed women i can do so in real Life, i dont need an RPG for that.
Thats my cents anyway