One of the biggest problems I've had so far was with one of my apps not working very well with Spaces. I don't know what happened, but it made my Maya window disappear. I relaunched Maya, and I lost a big part of my work. Fortunately I had a Time Machine backup, so I only lost a half an hour of work.

It seems like Leopard had a better release than Vista and I'm not having as many problems as those guys did.