The only bad COD was MW2.
Compare CoD4 to MW2 and Black Ops.
They're both bad and it's gone downhill. MW2 had a completely messed up gameplay whilst Black Ops had bad gameplay and technical problems (such as hit registration).
CoD hasn't gotten its standard (or ACTUAL competition) since CoD4.
I'd compare CoD4 to Bad Company 2, but that's quite of an unfair assessment, I'll stick with Battlefield 2.
Battlefield 2 has a multitude of classes, plenty of tactical options in large 64-player maps and allows your own play style to be successful if you're capable of following it up since situations quickly change.
Call of Duty 4 is a fast paced, balanced shooter that has a lot of maps made for different things, some quite obviously (hai Creek). Your strategy is usually the same, but the adrenline rush is what really takes over. Your weapon is simply best fit to how far you regularly shoot and furthermore your positioning in these small maps in order to get maximum effect. Furthermore, this has insane pro-competition, such as I40-41-42.
- H