Deniable Ops is the most fun I've ever had in a co-op game. There is a constant level of challenge of coordinating stealth takedowns with another player. The characters of Archer and Kestrel were arguably the most interesting characters in the entire game, I consider it a lost opportunity that they didn't expand upon either characters development. The story of the Deniable Ops campaign was interesting as a prologue to Conviction, especially the ending. Hunter mode was also fun and challenging, utilizing stealth gameplay to take down terrorists cooperatively.
Working with another player really made it seem like you were working with an actual TEAM, rather than just AI bots. The coop campaign was good at forcing players to cover each others advance, and just generally look out for their well being.
This is a game that absolutely celebrated the ridiculous friends/rivals mechanic that exists between cooperative players. The crude humor that infuses the interaction between the dynamic duo of Tyson Rios and Elliot Salem matched the level of absurd carnage they were inflicting. Combat was just an extension of this revelry. It's a shame that they neutered the humor in the sequel, and just tried to play it straight, unsurprisingly, it didn't win over anyone new, and it felt like a betrayal of the people who actually liked the first game.
Cooperative play isn't a suggestion, it's a must. It's a credit to the brilliance of designing a smart and aggressive AI Director to ensure that even playing the same campaigns over again never gets redundant, as the game works to keep you on your toes at all times.
This iteration of Halo isn't any more specially designed for co-operative play than any other Halo, it's just that I happened to have more fun playing it than any other Halo. The inclusion of Firefight yet another thing that set this game apart from it's peers.
Special Ops mode is the only reason I played this game. It turned an otherwise mediocre game into a pretty damn good one. Special Ops mode was consistently tense, fast-paced and quite inventive in their use of teamwork.
I enjoy playing the campaigns of this game cooperatively, if only to see the ways that other people tackle the challenges, and see how our gameplay styles differ. Again, the inclusion of Firefight mode is amazing.
Basically what I said about Left 4 Dead 2 can be said about this game, they're both stellar. The gameplay is somewhat better in the sequel, but I do admit that I love the characters of Left 4 Dead more.