COD is a game that plays off of false nationalism. The war on terror and so many US troops placed around the middle east and the world in general, the majority of COD players are teenagers drawn in by social media that modern war is exciting and you are being a hero.
Halo however, always played off the image of a universe that has many secrets and stories that have never been told and that is why we love the story. We also hate the flood because their presence in the universe and constant battle engagement through the series is not a slam on creativity of story, but rather keeping a great plot line growing. Man vs. Man and Man vs. Man's past.
As far as online gameplay, Halo takes itself more serious, because the controls are catered for players who want a lot of dynamic control and options. COD relies too much on killstreaks to advance games forwards and to shift moment in battles. Halo in comparison just needs some great communication between players, and strength in each maps unique weapons e.i. Sniper, Rockets, Pro Pipe and how to create the choke points to make these weapons effective.