I have played the earlier Call of Duty games. They were a lot of fun but became very "same-ish". They weren't very replayable. However, after playing CoD4 I immediately started again....and again! It is just as much fun every time.
It has an excellent story line. Surprisingly (and pleasingly for us Brits), the lion's share of the action is with the SAS. The acting is good (and amusing in a non-politically correct fashion at times).
I was stunned by the graphics. They are almost cinematic. Of course, graphics alone don't make a game but in this case it helps immerse you in the story. The realistic scenery, coupled with the physics make it feel real.
Bullets can now pass through some materials (eg walls). It's no longer possible for you (or your enemies) to take cover behind a sheet of paper.
There are two breaks from the all-out action. One is a stealth sniper mission. Normally, I find those pretty boring in a shooter game. Not this time. I loved it. There is also a mission where you act as a gunner in an AC130 Spectre. You sit in complete safety as you blast the enemy with an assortment of lethal weapons. Your only worry is blowing your own side to smithereens.
In the old days, I would always put tactical shooters ahead of FPS corridor shooters. However, the latest tactical shooters have been "dumbed down"(except, maybe Ghost Recon Advanced Warrior). CoD4 is linear with no direct control over squad mates. Nevertheless it's a great substitute for that feeling of reality that tactical shooters used to provide.