Highly-successful competitive multiplayer first-person shooters seem to be a natural "sweet-spot" for successful DLC sales, given the general dedication of the core audience.
Or, from another viewpoint, "The prevalence of de facto mandatory DLC in multiplayer first person shooters may result in the end of free expansion content in the subgenre."
There are always going to be the apologists saying, "You don't HAVE to buy it if you don't want to," but to that, I say "Try starting Modern Warfare 2 on a console without the DLC in August 2010".
Mods, sort of, were traditionally seen in PC multiplayer as a way to extend the life of a product, but... if you can SELL them that content, and it's first-party developed instead... and that's sort of the tinfoil-hatty version, but it's probably kind of true. -- Jake Rodkin, Idle Thumbs 48, 5 minutes, 53 seconds
And again, IW has not announced the price of the PC DLC (Robert Bowling's Twitter feed, the official community management source for Modern Warfare 2, specifically says "I don't know; announcement coming soon" on an April 6th tweet), but I don't think it's going to be free.