Do people like their interface to look more or less like a standard Windows interface, or do they prefer the custom look that most games seem to have?
I'll give two examples. First of all, let's say you have onscreen menu buttons, maybe they even appear over your game. If you make them look like a regular grey pushbutton, they do look kind of weird floating over a non-grey background. So in this case I prefer some sort of custom button look that looks pleasing over top of the game.
On the other hand, I have an in-game level editor with a toolbar. Right now, the toolbar is the grey type you might find in IE (on Windows, not on Pocket PC), with buttons that click down. It looks nice. It allows me to have colourful icons (over the grey background). But would such a standard looking toolbar look out of place in a game with otherwise custom widgets? On the other hand, let's say I gave my toolbar a custom "skinned" look, that isn't grey. Then, it might be harder to make colourful icons that look good on such a non-standard (non-grey) toolbar.
Same with the software keyboard. I started with nice grey pushbuttons with the pseudo-3D look, then switched to a custom look. They both look nice. I can't decide. What do you like?