This is my simple guide to designing a UI.

Keep it simple.
Make sure your design is uniform across applications, and especially within your own app. Hopefully even across different platforms.
Make your UI pleasant on the eye.

Make sure the user can actually use your application. Don't overdo the glitz. Make sure fonts you use are readable, as are the buttons and lables you apply them to. Make sure it won't take 3 minutes for your menu to pop up because of your drawing code. Don't use crazy wacky psycadellic colors, because it will be hard to reproduce them across platforms, and often leads to an ugly UI. More importantly, it leads to something difficult to replicate; which makes it harder when you're writing another app (or another part of your app). Make sure the user can get to where they want to get in your app quickly and efficiently; they'll put up with it the first time, if only for the eye candy, but after that they'll get tired of it. Don't overdo the eye candy. Put it in an easter egg or something.

I see so many applications that would be SO much better if only the writers had followed this style, so I decided to node it.