CAL4M1TY on 20 June 2009
Like Nightmare24 said, it's about being able to level up again.
Also, it's about new maps, new modes, new equipment, new perks. Even if this is just an update of CoD4, it'll be worth the full retail price (and not many game companies can boast that about sequels). But luckily for us, IW isn't treating it that way.
And finally, what you said about MW2 can be applied to any sequel that follows the gameplay and plot of the original. You are getting a new story scenario for Uncharted, a new story scenario for Gears, you're getting a fresh coat of paint on GoW from last gen. But you won't hear many people complaining about that because those games are worth it.
