Just read an article where they said BF3 can support up to 256 players in 1 map, but do to design & fun of the game they will limit it to 64. However, that probably means there will be some pretty big modded servers
Same article also said the new engine allows for terrain deformation in more realistic ways. Like they said if there was an earthquake you could see the ground separating and rocks rising up and shit.
They want the physics to be a more realistic version of bad company 2. When every game and their mother is making high quality graphic CoD clones, the gameplay really makes a huge difference. BF series always seems to know whats up when it comes to gameplay > graphics.
