If you liked CoD 4, MW2, MW3, or Black Ops 1 you'll like the new Modern Warfare. They all play somewhat similar in terms of mechanics, and the level design is back to the quality it was with those games.
On top of that, Infinity Ward has added a whole new level of weapon customization that is among the best I've played around with in shooters. It's very similar to the Battlefield or Siege customization, and has some unbelievable depth to it now. Attachments are more important than they've ever been.