As for the shows in the NYMF - Not all the works have to be new. There are some productions getting their first "major" production mounted during the festival, but a bunch of the works have been around for a while - maybe just not in New York (LA, Chicago, etc.).
There are also two distinct categories of mainstage shows. There is the "Next Link" series which focuses on newer shows, and the NYMF provides them with the opportunity to get the show on it's feet for the first (or second) time. There is also an "Invited" series which is comprised of shows that the NYMF staff has seen around the country. And as BK pointed out earlier, some of these shows have been around for years already. Heck, there's even a production of Six Women On the Verge of Braindeath (or Expiring Minds Want to Know) as part of this year's festival - and that show has been around for at least 15 years if not more by this time.
If the "ravers" on the chat boards can't read up on the show they're seeing, well... they don't read up on anything. Ah, well..