Speed is the most important feature. If your application is slow, people won’t use it. I see this more with mainstream users than I do with power users. I think that power users sometimes have a bit of sympathetic eye to the challenges of building really fast web apps, and maybe they’re willing to live with it, but when I look at my wife and kids, they’re my mainstream view of the world. If something is slow, they’re just gone.
Fred Wilson offers “10 Golden Principles of Successful Web Apps”