From a design perspective, I think the main problem most people have with mission limited content (such as zones) is the difficulty finding people who need it. This is certainly a valid concern - I've never had an issue with it myself, but it's undeniable that some people are just going to come late to the party and find it very difficult to find enough kindred spirits.

The solution, as I see it, is offering completion credit to players who finish a mission multiple times. A similar series of ideas is mentioned in Achievement Ideas Anyone. Basically, tangible and intangible cumulative rewards for helping other people complete missions. Certainly, I know if we had gotten GC seals for doing To Kill A Raven people would have been shoving to help out people who hadn't finished yet.