I think the excitement really ramped up after the multiplayer reveal, and they showed that they're bringing much more to the table than a carbon copy of the last IW game like world at war was. Stuff like the theater mode, wager match, currency and customization havent been done before in the series. And thats all on top of guns, maps, weapons and perks that we expect with each new installment. There was absolutely NOTHING worthwhile in WaW or CoD 3 that hadnt been done before and better by IW.
I have absolutely no problem with the annualization of the series. There really is a lot to the sports game comparison, but that only a loose comparison, because so much more changes between each version for CoD. Football stays football every year, but the game literally changes far more for CoD.
And the reason I believe CoD is still so popular is not because of anything you can list, but that intangible feeling of a CoD game that few others can replicate. The way the interface is so fast, the way you can get into a game before most other games are even done loading. The super addicting XP system. The ultra responsive 60 fps, the way if you put your sights on an enemy and fire it will hit, even when youre shooting at 1200rpm. Those who get into CoD MP big time really get their moneys worth - that $60 a year is negligible for the amount of entertainment you get for your dollar, and I'm psyched theyre putting one out every year instead of being forced to wait 2+ for each.
And if you dont dig the new version, no biggie....there's still thousands upon thousands playing the last few installments for years to come.