The online game play is where the true value in this game lies.
I too agree that the single player campaign is way too short, but in all honesty, they could make this game without any campaign built in.
The online gaming is where this game holds it's value, and will deliver to you hours and hours of deeply gratifying gaming. With all the variations of the online game modes, I doubt you wont find a niche you truly enjoy.
MW1 = groundbreaking fresh awesomeness.
MW2 = arguably better (I'd argue it's the worst of the lot) suffers from over the top "tactical nukes" that end the gameplay.
MW3 = has got the balance and presentation just right, and with the free Elite webiste, all the stats you could ever ask for (I like stats).
I think it's a shame people are only capable of all or nothing ratings, and this game definitely deserves more than 2.5 stars out of 5.