my 2 cents:
Action-packed FPS require younger players (teen to twen)
More tactical FPS adress older (30+) players too but the potential player base is smaller
Some (mostly older) players have a special relation to the WW2 setting since they grew up with a lot of movies about WW2 or even had relatives, who where contemporary witnesses. These players will leave the player base one day.
Then the question is, what will attract more players to a WW2 setting then to different ones?
Being strong at these differences will make a game or mod more successfull than others. Copying new releases like BF3 and such, hoping to profit from their marketing, will get you nowhere as they'll always do it better.
What makes the WW2 setting special?
Fighting changed from the trench warfare of WW1 to more agile tactics with smaller units working together, because the small arms got more effective while still not having the high precision of today's weapons. Camping and sniping is easier with todays weapons. In WW2 you still had to go into the action. You get the feeling that you have to rely on your abilities and intuition and that tactics matter more than sheer numbers, especially on maps with fewer players.
If we get the 128 players server it depends on how we play (it always depends on that). If we use the numbers to operate more tactical (have dedicated tasks: arty, spotter, mg's for suppression, use of smoke, tanks and inf working together, close air support, defensive lines with AT and AA) or just have a WW1 style meat grinder.
Another cool thing about the WW2 setting is the great variety of theatres and landscapes. In all places fights can get intense if weapons range is limited.
And the last one I can think of atm is that most soldiers were not professionals. While games about modern warfare present professionals as being either keen on following orders and fighting or at least cool-blooded killers, the WW2 setting has a special atmosphere of pain and sadness. War is presented as a dreadful thing and most soldiers long for other things in life and hope for the war to end.