As far as I remember, back around Oblivion Radiant AI was mostly cut out of the game because they realized that high end computers had problems running it with everything implemented. With no time to optimize, they cut out most of the system, leaving a core that was fairly reduced. It's a great idea, though, and not undoable - Mount & Blade pulls off stuff like that pretty well, with all npc characters picking from a varied list of goals, and creating a fairly believable environment and an interesting world that way. It's just about making a robust system of controlling the NPCs without scripting their every move. It's designing genius - once you have the system in place and working, you can rework it for different games, and get a much more interesting game world with much less work.
And, to be fair, it hasn't "always" been their selling point. It was one of their selling points for Oblivion, and I can't remember hearing anything about it in Fallout 3, although checking around, it's mentioned in interviews. I'm not convinced that the AI behavior will be much better than in Fallout 3/NV, but it will certainly not be worse, unless they fall into the same trap as in Oblivion of making 3 conversations for for the NPCs to pick from.
Stuff they must do for the game to work, though, is getting more damned people in the cities, making combat (and especially dragon combat) interesting, and make a decently compelling story. Morrowind had the first and third, Oblivion had, well, kinda the second, so they probably have it in them. Somewhere, maybe. Exploring the world will be good - Betehsda making exploration downright uninteresting would be like Sid Meyer making a game that won't steal away 5 hours without you noticing.