Lately I feel Treyarch's games are the only ones really taking strides in helping the series evolve in some fashion. Infinity Ward's last two games, while certainly not bad, were too samey. Modern Warfare 1 was a pretty big deal when it was released because it was a nice change up for the series (going from WWII, a setting that was considered overused at the time, and putting it in the modern day), and like it or not it did help to influence certain aspects of modern shooters (though not completely, I feel Halo was more of an influence [especially in the controls department, since it helped to modernize stuff like dedicated melee and grenade buttons]).
Modern Warfare 2 and 3, however, felt like they should have just been one game instead of two, especially given how they didn't really do much to try and change up the series (heck, even the HUD design in MW3 was exactly the same as MW2). Treyarch, meanwhile, have not only been giving us the tight controls and varied weapons and locales we've come to expect from the series, but they've also been giving us more interesting narratives and characters. Nothing against people like Soap and Price, but I found characters like Viktor Reznov, Alex Mason, and Frank Woods to be more interesting characters than most of the MW cast, and Raul Menendez a more compelling villain than Vladimir Makarov. Black Ops 2 also allows for the overall progression of the story and levels to be less linear than before, thanks to the fact the player can finally influence how the story plays out. The use of a near future setting is also a good change of pace, as it allows us to use weapons and tech that can keep some aspects of the gameplay feeling fresh.
I'll wait and see what Infinity Ward and Sledgehammer have coming up next for their game next year, but for now I'll more than likely just be waiting to see what Treyarch brings to the table in 2014.