A Fallout MMO has some great potential, but it has to be done right. The world has to be massive. Nothing is more irritating than having a post-apocalyptic world that's crowded. Any area that isn't a shanty city that manages to accumulate a large number of players will kill any feel to the world.

The turn based combat system can be done. With a world of the size it would need to be, combat can be done with the turn based system easily. Much like the original Fallout and Fallout 2, combat can be done in zones. As you're wandering the world and encounter an enemy, either alone or in your group, it could set up a combat zone border (probably given a green, transparent wall to show where it is), in which the player engages in the traditional Fallout turn-based game. They'll use their AP, the next player goes, the NPCs go, then it gets back to you, letting you use your AP again. Aimed shots take more AP than wild shots, much like the original game. Players crossing the green wall can get in on the combat.

I think it can be done. However, a Fallout MMO will have to make liberal use of ALL of the skills. Skills like science, which are best used for hacking, need to have a value beyond just knocking out robots. The world needs to be littered with bases, quests need to have a combination of people with great science skills, great medicine skills, barter skills and speech skills to complete, or at least get all the bonuses for completing it. They have to mean something. High speech skills get you access to better quests for example or boost the reward.