I have only played Fallout 3 out of those titles and to be fair, I had asked for sequels, not DLC, because I didn't play any of it's DLC either. For that reason I'll just use New Vegas.
New Vegas added a new story, a new map, survival features that are entirely optional and thus, don't really count for much if you ask me. Did I forget anything substantial? In comparison to CoD 4, MW2 added Spec Ops of which the majority could be played in SP or MP, it added a new Story which, in contrast to News Vegas, builds upon it's predecessor, added new Perks, new Maps, secondary weapons that are not pistols (which is a Game-Changer in MP) a abundancy of new Killstreaks and it also generally added new weapons. In list form:
New Vegas:
- New Map
- New Story
- Survival Features
Modern Warfare 2:
- New Map(s, although the size differences should be noted)
- New Story (albeit a continuation)
- A Co-Op/SP Hybred Mode
- Substantial changes to the Multiplayer formula
So, I summed up the MP differences (which I personally don't consider a necessity, that's nice of me considering CoD is mainly a MP game) and it still has one point more on the list. Now all you could really bring up are things that I may have potentially forgotten about New Vegas or the effort around making the singular featuresm which would probably be very debateable considering that we (I assume) both have no formal education or experience in Game Developement.
I agree that CoD is a cash-in, I just don't find this particular argument to be all that fitting. I would like people to get the right arguments out there, then maybe I'd have a chance at seeing another CoD I'd consider worthy of full-price, currently I wouldn't pay more then 20 bucks on release, which consequently is what I do and is also barely over typical expansion price.