Back in 2007, when COD4: Modern Warfare first came out, it was really an awesome leap forward for it's time. Think about it. Perks, attachments, different game modes. The campaign had a revering storyline, and was honestly better than Halo 2 or even 3. However, don't take any offense, this is just my personal opinion.
Infinity Ward really nailed it. Call of Duty 4 is what reeled many of the current COD players. After COD4, World at War came out. It was still good, however it did not really feel like an upgrade; more like COD4 with a World War Normandy setting. Moreover, I preferred the modern scheme. World at War did add a couple of new features, such as the famed Zombie Mode.
Then, in 2009, Modern Warfare 2 came out. And by God, it was AMAZING. Introducing Pro perks, Special Ops and much more. The amazing thing was the Killstreaks. MW2 redefined killstreaks for Call of Duty. Previously, it was set at 3, 5 and 7 (typically a spying UAV, an airstrike and support helicopters or dogs. In MW2, they added more types of helicopters, supporting fighter jets, nuclear warheads, EMPs, Care packages, SAM turrets and Sentry Guns.
Modern Warfare 2's gameplay last until a week before MW3's release. I ditched Black Ops for it, as it was honestly better.
Modern Warfare 3 used the same Engine, so the HUD and most other things feel the same. It's more new features. Splitted killstreaks into 3 categories.