The biggest part is the game's engine. Most post-WoW MMOs have used recycled engines from first person shooters that weren't designed to accommodate a lot of players (hence the "invisible armies" problem in GW2, for instance); but making your own engine is expensive and takes time... so I'm not sure what Mark Jacobs is planning to do. In any case he should be aware of this problem, so let's see what he comes up with.