From hardcore gamers, to casual gamers, Call of duty 4 pleases the entirety of every crowd. How you ask? That's simple, it involves one thing no other game has - Well implemented role-playing elements. I understand fully that other games may have these elements, but are they well implemented and surrounded by a game of great detail and action?
I recently went out and bought Warhawk for the Playstation3 for the sole purpose of being able to play with my friends online at the same time, from the same console. But after a few hours of play, it became dull compared to Call of Duty, and we began taking turns again on Call of Duty. While you CAN edit the colors of your ship, and download many different expansion packs, it always ends thinking "Well that was fun, I just lost a few dollars."
In the terms of maps, Call of Duty still brings in a heavy hitter in that area. Having a large amount of maps already in use, they released a map pack that brought 4 more maps for use in online play. Too bad you can't use these maps when you aren't connected to the internet though. In any case, COD4 still stands out from the crowd here.
COD4's physics are mostly correct. Other than being extremely far away from someone and having a sniper bullet hit them .50 seconds later. Playing in hardcore mode gives you the most realistic experience in any game I have ever played though. Its counterpart, team deathmatch isn't very well-balanced, as juggernaut perk hurts the fun value, just like martyrdom hurts the fun value of hardcore.
One of the very few problems I have with Call of Duty, is the online community. Being unable to mute people while the game is counting down to start is really irritating and should be fixed. People like to play music through their microphones too much.
One other problem is the Single player campaign. It felt sort of thrown together and bland. Not to mention it was extremely short. It made you think Infinity Ward's one and only focus was multiplayer, and single player was there as an extra.
Call of Duty 4: Modern Warfare is almost perfect in every way multiplayer-wise. With the addition of an online co-op mode, I'm very confident it would further its lead in the FPS market. If you're on the edge about buying a good multiplayer game, and you don't already have Call of Duty 4, take my word for it, and go get it. Even if it's a 2007 game, it brings intensity and graphical prowess of a late 2008 game.