Demize99:First, the 4 on 4 TDM setup is for Gamescom's lounge specifically. The full game will support 24 players in TDM by Design. In theory we could go bigger on PC of course, but we wanted to focus on a tight infantry TDM game. There are no vehicles in TDM.
Second, on Motion Sensors. First, you cannot throw them like the motion balls in BC2. You deploy them, they are larger (and easier to see) than BC2, they can be destroyed, they beep when they detect movement. A recon soldier can only have 1 deployed at a time. You can crouch or prone and move past a motion sensor without setting it off. Motion sensors will only show you briefly on the minimap (approximately 0.5sec).
You can, therefor, easily walk into range of a sensor, get detected, crouch, sneak away and either destroy the beacon or move and ambush anyone nearby who goes to investigate your position.