Late 1960's. CIA. Cuba. Vietnam. Cambodia. Soviet Russia and China. Pretty damn fun. I just played this a month or so ago. I'm sorry that I didn't get on the boat with this one sooner. Now to play Modern Warfare (I haven't played any of those) and see what I've been missing.
Developer Treyarch has a good record of keeping things fresh in Call of Duty. The company started working on the franchise back in 2005. With World at War it added zombies; Black Ops went to Vietnam; Black Ops 2 traversed time and added branching narratives. For its next installment, Treyarch is, once again, trying something new.