Thinginess

When considering different visual UI designs, a heuristic I use is "which thing looks more like what it is?" E.g. which button looks more like a button (the way a user would expect a button to look)? Which input field looks more like an input? Which scrollbar looks more like a scrollbar?

This can be considered a kind of deference to convention and perhaps a degree of skeuomorphism. But a UI that is not legible is certainly not good.