At the end of the day, I play the Halo series for the story. I feel that I only really need one "up to date" FPS in my collection at a time, which is why I don't own any of the iterations of COD or Battlefield.
Well, except for COD 2, but that was from a garage sale for one dollar. Intense mouse gripping action there in single player.
Anyways, the main reason I have an up to date FPS at all is because the online multiplayer seems to be better in the more recent ones. That is, better features and better support. The second reason is I need to have at least one of the titles that my friends like to play so I can play with them. Fortunately, they're mostly Halo fanboys so my pick of FPS goes. I just wish they'd stop mocking the other versions of FPS out there (Yes, I do think of all FPS's as versions of the same game)
The only way that 343 can really get some nerdrage out of me is if they screw over the storyline and characters. For instance, in the first trailer of Halo 4 that ever came out, Cortana kept calling Spartan 117 "John", even though I doubt she has ever called him by his name before. I'm willing to let that slide because Cortana recently had a serious issue with sanity when she was captured by the Gravemind, but if there's too many of that sort of thing going around, I'll get annoyed into sending a thousand page letter to 343 explaining how they went wrong with the story.
See, 117 is a badass supersoldier who has no face. In the fan fiction, I'm fine with them speculating on lost loves and facial features all they like, but in the games he should be the "Hyper Lethal Vector", same designation as Noble 6.
That leads into the next point nicely. I personally would have preferred Chief to float in space a while. In the meantime, the next generation of Spartans and ODST still have a fight to finish, mostly with the Brute led Covenant remnants and even with the Elites over Forerunner artifacts. In fact, that would be a nice new direction to take us, more spec ops and less 'I'm going to blow up this ring and everything on it'.
I get ranty sometimes. I'll leave you with this
EDIT: One last thing, the DMR was nice for me. It was like having the old pistol back, only it made sense. I have a lot of sympathy for 343, they've got a huge challenge to keep the game awesome and make it awesomer while making enough people feel that they stayed true to Halo. I hope they get it right, and I hope that the flagship game for the Xbox 720 (or whatever silliness they call it) is in the Halo franchise.