kz and gears had years between their sequels....COD has a new game every year....sure, they just swap from WWII to modern conflicts, but they are exactly the same games
Killzone had like 4 years inbetween. Gears only had 2.
Also Each year a different company makes the games. Infinity Ward, the makers of the MW series make the better series imo. They are about to get aw3ay from COD and just name them Modern Warfare (can't remember where i read this).