Character skill over player skill.
As soon as something is player skill rather than character skill (Like Skyrim lockpicking) it is not RPG.
A pretty hard-core statement.
But if it is true, then I prefer the action-game direction tes is heading at, as long as they keep up with the huge and detailed worlds. I'm a pen and paper nostalgic, but I don't miss the numbers. I miss the Dungeonmaster when it comes to 'true' roleplaying. Something I would never ask for as long as TES' ai's are dumber than my tv-remoter
But in todays CRPG's, variety in character creation, responsive surroundings and some game mechanics to support my role-based choises during the game, should be essentials. If people think any of this works in Skyrim, I envy them. I think it svcks big time.
But Skyrim is a beautiful, enjoyable action-hack&slasher, and I love it for that.
A knotty question OP. Ask 5 different RPGers this and you'll probably get 8 different answers. I'm going to suggest the basics are even more simple than Andaius has put forward. Levelling, looting, and the base ability to define your character.
I was idly considering this today on my way home from work, and, pondering, I feel the numbers are a hangover from "ye olden pen-n-paper" days, when the numbers were your only connection to a character. As technology has advanced, we went to CRPGs, but still, there was a 16 colour palette and the pixels were as big as my thumbnail!
As we've travelled down the CRPG route, we've become more "connected" with our character in ways other than a set of numbers. Our primary connection now is via a detailed simulacra on screen and our controllers, which give us free roam access to an entire world, thus making some of the numbers seem a little superfluous. It's a visual experience and connection as much as anything else now.
I think many of the people here think that Morrowind is better than Skyrim because they're wearing the nostalgia goggles, and I would suggest that for a lot of them, it was probably their first CRPG. Ah, you never forget your first time....
True!
But it is not just nostalgia. Morrowind offered more choises in character creation and the gameworld more room for roleplaying. It was simply better. Period.