Call of duty.
Granted, I picked it up at #4, but it wa a different sort of first person shooter, focussing on military trappings and realistic settings, with a morally grey story, and hard questions about heroism, interventionism, and nuclear weapons.
MW2: Oh look, that series that focussed on the British SAS? Well now America's invaded by the Russians who have all signed up with the Ultranationalists now. And I'm just going, really? It was still fun, but the only meaning you could take away is that you can't trust the military to regulate itself.
MW3: And then it got shorter and more linear, and the conclusion to the story is... A heroic sacrifice by a squad of US Special Forces operatives? I mean, really? The first game had a protracted sequence decrying movie heroics in warfare, and we end on that?
The story in 2 and 3 was a convoluted mess, with terrible exposition, and pacing, characters whose interactions and motivations seem to be invented fully formed depending on what serves the story at that instant.
Oh, and the Multiplayer:
MW: Mostly sprawling maps, with diametrically opposed spawn zones. Good for any number of play styles. Gameplay is medium to slow paced, lean is still a thing, and killstreaks are a relatively subdued affair.
MW2: Still plenty of sprawling maps, but now with some better designed close areas. Dedicated snipers can still have their fun with this. Gameplay is much faster, to the extent that "knifing" becomes a thing. Autoaim on snipers makes "Quickscoping" easy. Poorly designed perks lead to constant tubing. No Deds. Killstreaks are constant and bombastic. Things are going downhill.
MW3: Entirely close quarters zones for the most part, and using weapons apart from close in gear is futile. Game is fast, quarters are close, and akimbo machine pistols dominate all engagements. The maps now have ambient gunfire and floating ash effects, a nightmare for competitive players. No ranked Deds, and the unranked ones were buggy for the first month or so.
