Your idea, though nice, has a "big soft" problem! More often than not, the person who asks the question does not tag it "soft". Indeed, I have just checked the soft questions that are still alive on the main site, and I have realized that many of the nice ones haven't been originally tagged as a "soft" question. It seems that the "big list" questions suffer from the same problem; after all they become a "big list" after a while, and not immediately!
Update: Before getting more down-votes, I'd like to be clear about my stance. I don't like the idea and I think it is against the MO's spirit and motto: It's built and run by you. I strongly believe that "you" means "the users of MO" not the users of MO and meta.