The benefits, in a nutshell, are the ability to stay on-site in luxury accommodations at an affordable price. That's the reason to buy
DVC, everything else is pretty much a perk and can go away at any time.
There is a discount on AP passes of about $100 per person. There are no "perpetual FastPass" benefits. There are some discounts and other perks like pool hopping and free valet parking at DVC resorts.
Here's the list of types of benefits from the official Disney DVC site:
http://dvc.disney.go.com/dvc/guest/about/guestAbout?id=MemberBenefitsPage