Ever play the early Might & Magics? I managed to wade through M&M III. That was first-person, involved weapons, and was turn- and stat-based. (Wizardry too, for that matter.) They weren't even 3D.
It's still totally obvious that FO3 will be 1st/3d person real-time; an action RPG with gameplay much like S.T.A.L.K.E.R. if we're lucky. What we don't know is how deep the writing and RPG elements are going to be; that's what will make or break the game -- and what I'm most skeptical about. The trailer sets a pretty nice baseline for the atmosphere -- but we still don't know if it'll be genuinely fresh or just pastiche.