my wife finally broke down and asked for a kindle. it was a friend - not me that finally convinced her to take the plunge and she loves it! i had taken the plunge a while ago with the kindle app and i think this emphasizes one point on this list - an app that can take many forms on different devices will have more of an impact than one tied to a particular device or form factor. kindle supports such a wide variety of devices - from various manufacturers and operating systems, that it's easy to see why it's become the new mobile companion of so many.
the fact that there isn't a color model will certainly be corrected and we've seen the rise of other devices that address is, but from an app perspective, the kindle software makes my list this year.