My take on it is that it also has to do with narrative storytelling — roughly analogous to the workflow and how people discover their way through the UI.

Storytelling is a major part of a software’s appeal. The iPhone sticks to well-known tropes. Both Android and WebOS throw in the kitchen-sink for plot-line — and it shows.

This is what Apple’s good at. Your typical Mac doesn’t have a lot of extra buttons and labels on the front (each button represents a potential “subplot” in the narrative.) The result is that the user quickly feels comfortable with the iPhone. There’s comfort in knowing that pushing that Home button will always take you back to a known place.

With Android and WebOS there’s a lot of stumbling around and an undertone of uncertainty, like you never know what something will do if you tap, swipe, or push a button at any given point.

That certainty/uncertainty issue, I would wager, has a lot to do with a user’s ongoing sense of ease and satisfaction with a device.

Good animation is important, but so is a good story.