Fallout 3 and Fallout New Vegas are the best Sci-Fi RPGs I have ever played (and I am still playing through the various pieces of DLC for Fallout New Vegas having recently finished Old World Blues and started on Lonesome Road)
There aren't too many things that will make me not buy this. If Fallout 4 on PC doesn't have the awesome mod support Fallout 3 and Fallout New Vegas have I wont be buying it. If they add any extra crap DRM on top of the Steam DRM I wont be buying it. If they make the system requirements too great that my fairly beefy system can't play it I wont be buying it. And if they do anything to intentionally make it harder to reverse engineer the games data formats and stuff I wont be buying it.
Oh and they should put some effort into making the engine more stable and less prone to crashing (Fallout 3 and New Vegas aren't exactly the most stable games I have ever played)
Not too sure I like the idea of randomly generated dungeons or quests either, I much prefer the hand-built dungeons/quests of Oblivion, Fallout 3 and Fallout New Vegas to the randomly generated areas of games like Diablo 2.