Treyarch puts a lot more thought and care into their games. If you take a look at the differences between MW2 and MW3, you will see that there aren't that many. In fact, you could call Modern Warfare 3, Modern Warfare 2.5.
The differences between Black Ops 1 and 2 are much bigger than past installments. While being a direct continuation story wise, the series has bad it's way into the future and evolved into something great. While the method of unlocking has changed only a little, the ways that you can play the game are only limited by the Pick 10 system. Even in the campaign you are given larger methods of choice in the story and what you go into a mission with.
