I really enjoyed your assessment, nydirector, and I think you hit the nail on the head when you said that our ability to understand the jokes is the key to enduring comedy. Real humor, I think, translates to understanding the elements of absurdity in the human condition. It doesn't change over time, really, which is why Aristophanes, Moliere, Shakespeare etc. can still make us laugh. Human nature doesn't really change that much. However, I still contend that truly topical humor, which relies on specific historical or political situations to get a laugh, may not fare so well in the long run. For example, a person can watch a "Carol Burnett Show" from the 1960's, and because it's based on character-humor, it's still funny. However, if you watch something like an old, "Smothers Brothers Comedy Hour," where the humor was rooted in picking out the absurdities of that particular time in US history,(without applying it to the situations in 2005 America), the humor is dated and a bit stale.
The funny thing is, when those shows were new, I was fonder of the Smothers Brothers than I was of Carol Burnett, but however broad the humor was, it was character-driven, and thus, has held up better. I think this is also true of Broadway shows. Those that rely on a particular, cultural phenomena to relate to the audience, rather than to character development, won't be here in the far future. Of course, I won't be here to know about it.
As you've said, how the collection of characters is presented to the audience is in the hands of the directer, producers and cast. Any show will only be as good (or as bad) as they are. If they don't have a clear vision of their characters, neither will the audience, no matter how "timeless" the story. Even more-or-less current theatre bases itself on older tales. West Side Story is a reworking of the Romeo and Juliet story. Everyone knows this. Rent is a reworking of La Boheme. Even the current, Woman In White is based on a Nineteenth Century, Wilkie Collins mystery. These stories weren't new when Shakespeare, Puccini and Collins constructed their current versions. I believe if you look back at the musicals we've been discussing, those that are 40 or 50 years old, they are based on a tale that is older, still; some version of life that is recognized in virtually everyone's collective consciousness.
I know this is long-winded, and for that I apologize, just wanted to put in another quarter's worth to the discussion, which I'm enjoying immensely.
"And the postman sighed as he scratched his head, you really rather thought she ought to be dead..."