The campaign is my favorite part personally. The story is much more personal. Honestly I forgot the other games even had a storyline, but that's what this one is all about. Sam's personal life and revenge for his daughters murder. The slowness of the past games is gone and you're more like a predator. Riddick meets James Bond. Unfortunately it's impossible to beat the game without killing (some people used to ghost the entire game with a 0% kill ratio except for bosses) and a few of the levels ditch stealth for a more cover-based gears gameplay, but it doesn't last for very long.
Co-op is a lot like Chaos Theory. You have a handful of unique levels and they have their own Storyline that's just as good as the main games storyline. You don't have all of the co-op moves Chaos Theory had, but you still work together to overcome your obstacles.
Multiplayer sadly abandoned the competitive Spy vs. Merc mode, but the multiplayer it does have is still fun. It's more like GOWs horde mode or Halo's Firefight mode, but it feels like there's more of a purpose to it. There's also Dark Ops mode, which is pretty much missions that didn't fit into any storyline. They just present challenges for the player to overcome. They were probably the best part of the MP offering.
Gameplay-wise, like I said. It goes more predator than stealth. You have a large variety of weapons at your disposal. You still use shadows, you still scale walls, etc... This time you can mark enemies. Whenever you get a melee kill, you have the ability to lock on to 3 or 4 enemies from hiding and executing the attack will go into a quick cinematic of Sam taking the enemy down. Kind of like FO3's VATS system, but actually cool looking. This time you're forced to kill instead of knocking enemies out, but not really a big deal to me personally. Like I said before, some of the levels get shooty and abandon the stealthy part, but overall still a really good experience. It and Chaos Theory are definitely my two favorites in the series and I can't wait to see how the next one is.
As for COD:BO: