After having played them so many times I wonder to myself "What gives FO games such great replay value?"
The answer for me is "S.P.E.C.I.A.L."
This simple system allows for so much character variation and specialization that it is impossible to make a character that is good enough to do everything (you could make a character that is kinda mediocre in a broad selection of skills, but even then you miss alot of stuff late game). So in order to experience an FO game to the fullest you had to play through it with multiple characters.
This is why FOOL is gunna be a great game.
No-one plays an MMO so they can play by themselves. Sure there are people who solo, but why would they play an MMO when they can get a much better single player experience in a single player game? No, they're just uncomfortable with the idea that they want a little social contact in their games. What better system to have in an MMO then one that requires you to cooperate with other players in order to experience the game to the fullest?
In other MMO's you have to group up to handle Bosses and Dungeons. Because of "S.P.E.C.I.A.L." you might want to group up to do pretty much everything. Because (This is all speculative ofcourse that the developer's stick to a system based on special and the base skills) you're gunna have a hard time convincing the elder to let you take the job that is the most worth-while if you don't have someone with good speech in the group. You'll also have a hard time finding the place if you don't have someone with outdoorsmen in the group. You'll certainly have a hard time defeating those raiders if you don't have a few people who can shoot straight in the group and you're certainly not gunna open the locker containing the uber tech gizmo you've been searching for if you don't have someone in the group with lockpick (or ungodly strength and a crowbar). If you want to sell that tech gizmo for all it's worth you also need someone with good bartering skills and if you want to stitch up that gun shot wound without visiting the local doc you're gunna need some medical supplies and someone with doctoring. You kinda get the point. If everyone in the group above has focused on one combat skill (i.e. Small guns/Energy wep/Melee) then you're looking at a group of 3-4 (based on the number of different skills needed to do all of the things above) players to complete this mission to the fullest.
The system that Fallout has ridden on since the beginning is an amazing system for multi-player games and gamesas knows it (I think Bethesda knows it too, which is why they made such a stink).
Stick with "S.P.E.C.I.A.L." and good ol fallout humor/story and FOOL is gunna be the best MMO in a long time.
If anybody cares to account how many times they've played FO2 go ahead. My number is 6. Another drunk conquistador conquering the governor's ball...