The two biggies are free parking and PARK discounts on food and merchandise. I've had my PAP for five years now and always take advantage of the discounts. Universal is much better to their passholders than Disney since Disney doesn't offer the park discounts. Also...Universal allows passholders to purchase tickets for friends/family at a discount...sometimes as much as 50% off.
While I admit those are the big perks, I find I really enjoy the little perks, as well. They did at least three free movie screenings for APs this Summer (I saw Hulk for free hours before it officially opened). When they previewed the Simpsons, the park was only open to APs (ok, so just Simpsons and MiB, but still cool) and they gave everyone a free "Squishee." During the first night of Mardi Gras, there was a special area APs could get cheap beer and they threw in a couple extra strands of beads for walking over there. Even the respectful way they ended Kong and BttF. It's the little things that make me put up with the parking problems and broken walkways and everything else annoying.