I’m a fan of Pandora Radio. I really love the product and I complete dig the simplicity and slickness of their Adobe Flash GUI…with one exception. I have an issue with the play/pause control. 
Pandora has chosen to recognize the current state of a song (playing, paused) by highlighting the associated button. The image to the right, for example, shows the display when a song is playing. Makes sense, eh? Well, yes, this strategy makes complete sense until you consider how users are going to interact with the application.
Quick. Let’s list every possible action a user may take while a song is playing. Pause the song, forward to the next song or adjust the volume. That’s it. What action will the user absolutely not take? The user won’t play a song which is already playing. Never. It just doesn’t make sense. However, since the buttons serve a multiple purpose (display state and trigger actions), the user is presented with a prominent, highlighted, active play button when they wish to pause a song. Even worse, the pause button appears to be disabled. If the UI could speak, it would be screaming at the user, “The song is playing? Clicking the play button is your only option!”
I’m of the opinion that Pandora’s choice of button highlighting and “multi-purposeness” confuses things. Is it just me or is this simple interface too simple? Consider user interaction.