Okay, they all have a different single player campaign.
for example
halo has constant powerups (I guess you can call them that), vehicles, large maps, and high character and map customization
whereas CoD has their killstreaks, smaller maps that are easy to get around on foot, and high class customization
for some reason they even took out the only character customization they had. but w/e
there's also a lot more that's different