An over-reliance on a hover state leaves tablet users in the dust, though. I see no reason for completely flat buttons, as it's eminently possible to design ones with subtle depth that play well within the bounds of "distinctly digital". The principle that your users need to touch everything in order to learn their boundaries is a dangerous gamble, and may very well be leaving conversions on the table due to risk-aversion.

The new Blackberry OS is like this with gestures. Sure, you can spend 20 minutes moving your finger(s) in every way imaginable to learn how to use it because there's no chrome (or hardware buttons), but it certainly doesn't instill confidence on the first use. Instead, it presents itself as a challenge: "learn how to use me, I dare you."