I'd say the Four archetypes of RPG's are:
1. The rigid linear story based JRPG (FF, Chrono Trigger)
2. The more nonlinear open world adventure games that haze the gray area (Deus Ex, Mass Effect)
3. And the Boots to satchel, Tabletop origins, tell your own story, build-a-hero workshop RPG's. (DnD, TES, WoW, (Fable?))
4. Crap (Fable III?)
There are categorical issues with the above (MMO's place in the scheme of things), but it points out the fact that a RPG has been a definition of widening berth over the years.
Now there are several definitional paramaters of an RPG. Customizabe character development, equipment managment, etc. But the most important to me is the R and the P.
Role playing to me is imagination. You have to imagine these characters from ES and Fallout are doing the things prompted. And bethesda, i believe does an outstanding job of giving us the tools to play a role
Sure you might have to limit yourself from doing things that aren't appropriate for your character, because nothing is locked out due to your choices. Some might see this as a con, but that also leaves you open for your Archmage to grow wary of his collegiate duties and explore the darker sides of a drug addiction. Three months later he's a skooma addicted vampire that is pounding his way through the assassins guild on a grudge. AND he can still summon a blizzard at will.
I Implore people to consider any game with that much range. The limits of the Roleplaying in any ES game is only up to your inner child.
And i would be hard pressed to see Skyrim as anything other than an RPG.