The game is immensely satisfying on a moment-by-moment basis. Infinity Ward knows how to create moments. They can’t very well create tension. Yet for memorable moments, Modern Warfare 2 can’t be faulted.
Everything else, however, falls into generic shooter territory. When the barest strand of plot links your story together, there’s a problem. When it feels like your game is a really pretty version of Doom in a modern setting (sans demons), there’s a big problem.