I think Don Norman's book "The Design of Everyday Things" is largely about exactly this, and it could even provide a simple heuristic for whether or not a speci

I think Don Norman's book "The Design of Everyday Things" is largely about exactly this, and it could even provide a simple heuristic for whether or not a specific skeumorphism is worth keeping or just kitsch: Does the additional design (whether a gradient, bevel, shadow, etc) indicate an affordance? That is to say, does the extra stuff that is being layered on serve to tell the user something about what the UI element is for. For example, a slight gradient and bevel on a button informs the users that it is meant to be pushed, as it looks a bit raised up relative to its surroundings. This, to me, is the biggest thing missing from these flat designs. There's nothing left to indicate whether something is a button, a panel, or whatever else. It's been stripped down so far that crucial UI hints have disappeared. As other commenters have pointed out, I think moderation is the key.