Just me, but Fallout seems like a single player RPG experience. Roll in coop maybe, but why?
My experience with MMO has been good, but you know why? I tend to run lone wolf in any MMO. If I join a guild it is always a guild of mostly lone wolf players. The value of a MMO to me is a well-maintained game where I only have to worry about having a client on my disk.
Play a lot of WoW early on. Got my fill of raiding in those early days. My most recent return to WoW was over a year ago, started a Night Elf Hunter, got him up to level 71 in two months, elite flying mount, not rich and famous, but rich anyway. Never grouped, never ran an instance. Found a nice guild of like-minded players. Lots of fun chatting. Good folk.
I have 7 classes fully leveled and equipped, all their primary skills and such, in Guild Wars. Not rich and famous there either, but rich! I still go my weekly round of activities. Great game for a lone wolf now that they have the hero and henchman system fine tuned.
Guild Wars 2 is looking very good so far. That is the only MMO I really look forward to, and I am looking forward to it a lot!
But the Fallout motif as the basis for a MMO? Somehow to me it doesn't really seem to fit. I do like Fallout, but I do not see myself playing a MMO version.