WoW did it all later than Arcanum. It's just a mainstream MMO that added nothing to the industry.
Arcanum is wildly different from Fallout because it greatly expanded its gameplay, using story and setting. You can only branch out so much in Fallout, and neither branch has any great disadvantages compared to advantages. In Arcanum, you usually follow the path of magic or technology - meaning that eventually you see the impact your choice makes on you. Technologists can't wield magic items, and the party healer won't be able to help them. In turn, mages have to rely on only themselves, and cannot make much use of the handy items the technologists craft from various useless trash. "Magic Versus Technology" got turned into a powerful gameplay element, and that's no small feat. Speaking of which, Shadowrun should also be on the list. Nobody said the list is just for COMPUTER games.
Finally, there's also complexity. Diablo 1 and 2 are both great games - but D2 added very little to the concept, was even in the same setting, and yet it is still a must-play game. Even I can acknowledge that, what with my dislike of Blizzard and all. It was simply bigger and more detailed, and by that it earned the respect it deserves. Arcanum relates similarly to Fallout 1&2 - it's more complex and more detailed. Though I haven't explored every corner of the map in Fallout, my expeditions around the various places of Arcanum were nothing short of mindboggling.
ninja-edit: Fallout is similar in many ways. SPECIAL, unless my memory completely fails me, was still used. The overland travel map was largely similar. And, of course, the developers, the legendary Troika Games, have made them both. They also share the greatest feature of all - after you complete the game, you are told of the aftermath of the choices you made throughout the game, sometimes even those you might have considered absolutely insignificant.
ninja-edit-edit: Oi, the combat system was also the same - you just had the option of going realtime.