More Call of Duty 4: Modern Warfare details

– co-op Squad Mate mode is drop in/drop out
– different colored reticle for your squad mate
– no shooting for second player when zoomed in
– same experience system as 360/PS3 version
– 10 players online
– host migration to keep the game going if host disconnects
– import your address book to play alongside friends without having to put friend codes in again
– better textures, fire and explosion effects
– fully customizable controls, a decision influenced by The Conduit
– consistent 30 frames per second

Source 1, Source 2