I was just reading an article on this.
Here are some things from the article:
"Now part of a new commando unit, MacTavish will guide players through a series of grueling missions, including an ice-climbing infiltration of a frozen Russian army base."
"Those looking for a single-player campaign longer than the original's 6-10 hours are out of luck. 'Stretching out the story seems like the wrong way to go,' Infinity Ward CCO and president Jason West told Game Informer."
"However, the cooperative campaign that many had hoped would be in Modern Warfare 2 is MIA. "We did a whole lot of co-op stuff early on that we were interested in, and it didn't work with the story at all," West said."