I don't really have to emphasize that combat should at least be near the standards of shooters like MAG or even the cream of the current shooter crop, Call of Duty. Obviously, with the wide open world that a Fallout game demands, some of the fidelity seen in arena-style shooters is going to be lost -- but remember that we're talking about an MMO that's probably not going to see the light of day until sometime around 2013-2015. I'd like to think that by this time technology and game design will have reached a place where this (pipe dream of mine) is possible.
Speaking of large zones, my Fallout MMO would ideally have pockets of heavy content -- like cities and strategic resource areas -- separated by a carefully constructed amount of wasteland. The tricky point with an big world is making sure players don't get lost in those more desolate regions where exploration can lead to cool finds, but also to mind-numbingly long hauls through endless terrain. Simply removing large expanses from the game is unacceptable, because it's part of the package with Fallout. Also, we already have plenty of theme park worlds to explore. It's time for someone to take sandbox design to the next level, even if it takes four or five years to happen.
As for how best to direct players across those vast lands, remember that Fallout is set well after the nukes have dropped. This means that society has begun the slow process of rebuilding itself -- in a fairly messed up manner. Still, road signs, maps and a solid understanding of where high population numbers are would be common knowledge. What that means is: Every player should start with a basic-yet-informative map that can grow if they choose to explore the blank regions of the world. This would be a feature I'd like to see developed up a bit into a really slick interactive map that players can write on themselves.
And speaking of the world, my Fallout MMO would have lots of scenic variety. Just because the world was charred doesn't mean that beauty wouldn't have thrived in strange and exciting ways. Some parts of the world aren't going to have been blown to high hell, much like Vegas in the upcoming Fallout: New Vegas. This is also a chance to see other places in the world like Europe, Asia or South America. Hey, what's going on in Southern Africa? Let's find out already.