COD, all of them, and HALO, all of them, ruined every FPS I've ever played since. And I mean that in the very best of ways. It even ruined Half Life for me. (Don't hate me, it's just a taste thing.)
I was a Medal of Honor fan. Allied Assault got me into FPS and back into PC games for that matter. I played Call of Duty on a super-high-powered-$2000 gaming PC and the first HALO on an XBOX at Best Buy and I've been hooked ever since. I actually BEAT HALO while standing there, I went back to play it so often.
They achieve the exact same goals with completely different stories, characters, and situations; HALO with Sci-Fi and Call of Duty with WW2 and the modern era. You are a voiceless, and in the case of HALO a faceless soldier bringing your attitude to the situations you find yourself in. COD goes further by putting you in multiple person's shoes: Russian, British, and American. Both allow you only two guns which makes the game that much more intense because you have to choose what guns you carry into a particular battle, whereas with MOD games and games like BIOSHOCK you can carry six guns. With both types you carry an obscene amount of ammunition and get hit by bullets, grenades, and other projectiles more times than any human can realistically take. At least with HALO you're a shielded and armored solider so there's a bit more "reality" there.
The backrounds in the both series, HALO and COD, are gorgeous but, unlike some of the other games, the characters are just as highly detailed and interesting. There's an incredible depth, texture, and smoothness to everything around them. FEAR, which I wanted to play for the longest time, severly dissapointed me on this count when I finally played it.
They're also two of the fastest games I've ever played. You're always moving, blasting away. No puzzles, no bullshit, just pure A.D.D. satisfying action. The variety of the missons and everything you can do within them ruins the running around with nothing to shoot at of other games, even some of the MOH games I've played.
Everything about both of these series was top notch in every sense of the word and thanks to them, I can't play any other FPS without holding them to that impossible standard.
"Show me a hero and I'll write you a tragedy." - F. Scott Fitzgerald
(My writings and ramblings.)