Modern combat was very cool. But the room-to-room tactics with all the gadgets (as found in many Tom Clancy series) gets a little grating. It's like the difference between a WWII fighter sim vs a modern Jet sim.
Gimmie a Thompson to flank an enemy trench over heat vision goggles with three different settings, snake cams under the door before arming it with a stun blast to have your (half AI) squad declare the room clear.
Another thing... as much as I loved CoD IV, multi-player would do well with random generated maps. I hated how each map comes down to a very limited bandwidth of possibilities... play them and you'll earn kills, deviate and someone better at working the formula will rack up kills on you.