Modern Warfare 2 is a nice step forward from MW1 if you enjoyed that. Your guranteed an excellent 6-10 hour-long campaign with great cinamatics, in depth story and characters you might actually get attached to(I MISS GAZ :/).
Theres spec ops, a series of co-op missions including snowmobile races.
A whole new set of weapons, customizations including kill streaks this time, and theres other shiny stuff to be had, http://www.youtube.com/watch?v=-QMXZNwLnME , hope thats for the pc version aswell.
Theres a new graphics engine in place which again is nice improvement from COD4.
Ive never played Halo/ODST so I cant say anything for or against that.