It strikes me that what could well kill FO3 is not that they'll ruin the dialogue -- which seems inevitable, given the way Bethesda has always handled dialogue in the game -- or the combat -- which wouldn't bother me much because I've never been too beholden to a game's combat system -- but rather that they won't be able to nail the visual style / mood. That's part of, maybe the biggest part of, what killed FoBoS (aside from crappy gameplay, etc.). If it had had the look and feel of a FO game, it would've done much, much better, I think. Instead, it looked (to me) like a cyberpunk type game devoid of the 50's art / deco style.
One thing that's always disgusted me about Bethesda games is that while their technical execution of artwork is usually pretty competent, their visual styles are crapolla. It's not the uncanny valley, either. It's just really bad art design that makes their games look uninteresting and mechanical. I have a hard time imagining how they'll avoid falling into that same trap on FO3.