Like most Disney nerds, I'm just a little obsessed with crowd levels in the park. I just returned from a quick weekend trip to Disney World, May 15 - 17 and was pretty nervous about the crowds before I left since they were projected to be 8s and 9s by the Unofficial Guide, which I've always found to be very accurate. There's no question that they were lower than that, probably what I would normally expect for 6s and 7s. There were definitely lines for the major attractions and fastpasses were gone for Toy Story Mania and Soarin' by early afternoon, but overall, it was very manageable and some attractions were walk-ons.
Extra Magic Hours at Hollywood Studios were very light; Toy Story Mania was swamped, of course, but it was possible to walk on just about every other attraction. The Magic Kingdom was open until midnight on Saturday night and the crowds were very light, especially after 10:00 p.m. We were practically the last ones in the park when we left at about 12:45.
We were also able to get same day seating for California Grill and several popular restaurants in Epcot (didn't try our luck with Le Cellier), which probably is just a reflection of how bad the economy is than how low the crowds were. People are still coming to the parks, the buy 4/get 3 deal is keeping the resorts full, but they're not spending as much on food and, I assume, on souveniers. Keep in mind that just last year, it was practically impossible to get a reservation at California Grill a month out, so this is pretty major.
The only difference I noticed in the parks from last May was that the bathrooms were dirtier. I'm not sure if this was just bad luck or a result of cut-backs in the park, but most of the bathrooms were pretty appalling.