Probably nothing. As far as I know there are no moving parts in the phone and im sure everything on the circuit board is soldered or epoxied. I would think that as long as it turned on afterwards it should be fine.

 

No. Most solid state electronics must work perfectly for the device to function. An intermittent fault (from a cracked circuit board or solder point for example) would also become evident. You should know by now if you broke something. Compare this to a typical hard drive where damage may not become apparent for some time.

 

Besides just checking to see if the phone powers, check the camera. I dropped my iPhone down the stairs a few months ago and now the camera won't work unless I hit the phone on it's side a few times.

Can't wait until I can order my Inc.

 

Most (if not all?) modern smartphones don't have moving parts. They use flash memory, etc. Plus I'm pretty sure they do a lot of testing for this stuff. I remember when the Nexus One was coming out there was a video of all the stress tests they ran on it, including one that simulated several drops, a crazy amount of button pushes on the trackball, one that pushed on the center of the screen to show how much it could bend. Kind of a neat video, and since the N1 was made by HTC I would assume they do something similar with most of their phones, especially something as high end as the Incredible.