I find consistency easily the most overlooked aspect of video game design. It's also one of the most important as it contributes to the "look and feel" of a game.
For example, in Windows OS you know that the little red "x" button closes the window. You know how it looks and feels.
Now imagine you're making a game with several layouts. These share a similar "x" button. In one layout it's blue, in another it's yellow. In one layout it closes a dialog box, in another it maximizes it. I'm playing your game and my experience has worsen because not only I don't know how it's supposed to look but it serves different functions as well. How am I supposed to be well versed in your game if the UI is broken like that? It makes for a cumbersome experience and I won't bother playing it.
Consictency refers to other aspects as well:
Before I start designing a game, I think about the overall graphics style (theme). You can't have a layout with 8 bit graphics and another with isometric high res (well some top notch designers mix and match styles but I have a long way to go until I get there).
Or how about super nice angry birds style graphics mixed with pixelated low resolution sprites?
Is it really necessary to use 50 different fonts in a game? More on the subject, why one speech dialog box has different font than another?
I like to keep it simple. I choose a style and maintain it throughout my game. I make sure the the UI elements work as expected. Hell, I use an online colour wheel to check witch colours go well together (but that's another topic :-p).
These are all details. But they separate a really well crafted game from millions of shoddy games out there. So keep in mind, consistency is your friend.