Ozymandias has been working on compiling the best version he can of The Coop Bill of Rights, following Tycho's wandering into the subject earlier... Coop gaming is in the midst of a great comeback, a defining movement in the current generation, and something that I think a lot of people are hoping to see stick around and nearly become a "required" feature. Forget length or graphics, but "Does it have coop? What sort?" Hence the idea of a Bill of Rights to find a lot of good "rules" (of thumb, rather than of law right now) on what should and should not be required for any good coop experience.
I think that coop is something that really shows off if your narrative design really works well interactively or not. Easy contrasts are Gears of War and Halo 3... there are some key narrative moments that my brother and I can talk about in Gears that there aren't parallels of in Halo because I think Epic did a much better job at pushing more of the narrative design into the level design and making sure more of it "popped" in coop just as well (if not better) than in solo play.
Not to say that either has the best approach because there is still a lot to do in order to really draw cool and interesting narratives out of coop structures.
Anyway, just as interesting out of this discussion is my introduction to Co-Optimus, which is a site that has dedicated itself almost entirely to coop gaming across all platforms. I think if anything, such a "fan club" bodes well for the future of cooperative gaming.