GOOD modern combat system! I put down Halo 3.
Pros - A profressional level storyline, a immersive mutliplayer ranking system, weapon upgrades, "perk" upgrades (special abilities, such as run fast for longer, take more hits, deal more damage, last-stand), the ability to kill people while they are taking cover behind concealment (NOT COVER) - because now bullets will rip right through that house you're hiding it! General realizem with weapon functions, and sight-alignment for aiming. And lastly, but not least - the want, and need to STAY ALIVE rather then run in carelessly - will give big rewards, UAV call ins with 3 kills in a row without dying, 5 kills gets you a air-strike YOU can call in, and 7 gives you chopper support...
Cons - A short storyline, subjective and rude comments on multiplayer (as with any multiplayer game), no co-op (but that doesn't mean its bad.), abit laggy since it seems that a console hosts the game rather then they interconnect and communicate as such in Halo 3 - can make for a more laggy, unconsistant connection in game.
Over all the game is highly enjoyable, I've played it for over 20 hours, and its highly addicitive. Sad the story was so short, but you'll quickly leave that behind once you go online. Remember this key concept while playing - its very important... "Slow and Low". Call of Duty 4 wants you to treat the game as if you're death matters - which is hard to do but I'd say this game almost is a success at it.
Worth buying - you'll invest alot of time into it IF your a multiplayer guy.