256 players on a single server is hardly undoable as MMOs have been doing thousands of players on a server for years. Its not to say they couldn't do a similar thing with mag.
The reason 256 players is impressive is because real time combat with so many players is incredibly hard to do. MMOs do not have real time combat (Well, not properly atleast, until APB comes out and blows everyone out of the water, but even then it only supports 100 people per server... or was it 100 for one side, 100 for the other...)
???
Anyway, 256 is impressive, it's just that, there's some basic but not major structure to it. Gameplay demonstrations didnt really convey the idea that you would be able to actually SEE the spectacle going on, because everyone is just having their own little bouts.
It does seem very much like BF2 times 5, which isnt necessarily a bad thing (BF2 is an awesome game.) Player count doesnt matter as long as the game is solid...
sadly this isnt looking as solid as BF2. My guess is that it will be about Quake Wars level, which isnt bad at all.