While the campaign is short, the multiplayer is innovative and the overall experience is something quite epic.
The ingredients making up the final recipe in COD4 all add up in an astounding way: the graphics are great throughout, the campaign is awesome (though undeniably short), the multiplayer beats the "unbeatable" Halo 3, and the gunplay is well-executed and works well in the realistic environments and settings.
The campaign: A short, but brilliant romp through many environments and stages with a better-than-average and well-speculated wartime plot to back ti all up. And with the lingering threats of Veteran and Hardened difficulty (though I played on Hardened on my first playthrough...) make it easily replayable.
The multiplayer: Something really special. Work your way to upgrades and higher levels to improve your character in multiplayer. Yet it manages to be so well-balanced that even a beginner in a standard Assault class can destroy a level 53 that's all decked out. The battles feel real and intense, and the multiplayer is one of the only entries that manages to be realistic, tense, fun, and still retain that highly addictive quality that multiplayer games try to attest to.
Other: The arcade mode is fun for a while, though certainly I can say it wouldn't necessarily be missed if it weren't included. Otherwise, the voice-work, cinematic scope and realistic portrayals this games boasts make it something wholly unique.
Problems: There aren't many, truthfully; however, one glaring issue that could annoy many gamers concerns the respawn play in the campaign: one is not able to sit back casually and pick off and destroy a challenging room--there are no such unrealistic endeavors in this entry! One must clear a path and quickly and assertively progress before a new rank of freshly respawned enemies arrives! This is certainly a gripe of mine. Also, while the graphics are extremely nice throughout, there are indeed times of mild problem.
Then there are the typicals: the AI sometimes falters brilliantly, the level design can, at times, be a bit confounding, and there are the usual shooter graphical glitches and inconsistencies.
Still, most of this matters little, and the overall package is certainly something not to be missed by any shooter fan, or really any game fan in general. Highly recommended.