Sometimes, a simple setting change is enough to breathe fresh life into a series. Look at Crysis. The first game was set in a jungle, yet the second was in New York and the whole vibe of the game and the way it played changed. Resident Evil 5's Africa was different to RE4's Spanish-inspired nation, yet RE6 looks different again. It's the right move to do, and for fans who aren't completely sold, the campaign also has Cold War flashbacks to the days of Woods and Alex Mason.
3. The Unlocks
Something consistently great about the series is the variety of unlocks available for all aspects, especially in Black Ops. While some features, namely the face paint, were totally useless, they added a few things like clan tags and emblems on guns, new perks and the awesome ballistic knife. The thing that sets COD apart from other shooters is how the fast pace makes unlocking things rewarding. Yes, BF3's system was excellent. Every ten kills with a weapon unlocks a new attachment for that weapon, but once the tree was finished the weapon became instantly less rewarding. The constant flow of killstreaks unlocking mid match is a great reward for doing well, and plays on the strength of the game's longevity. Black Ops' varied camos and red dot sight design were so much fun and it added a lot changing each match to find the right one is engaging, and so rewarding to find the perfect one.