The new Disc Image Burning feature in Windows 7 is great... but the operation can't be cancelled. The Cancel button is there, but it's grayed out. There's also the X, but it doesn't do anything while burning.
Edit: By the way, the reason I consider this a UX quirk is because of the fact that both the Cancel and X buttons are rendered useless.
Update: For those thinking "Well cancelling the process would ruin the disc", yes, I know, but if you selected the wrong file to begin with, the disc is already rendered useless. Please read the comments for more information. Every other burning application allows the operation to be cancelled.
Allow the burn process to be cancelled with a confirmation dialogue.