Sometimes the background picture or video is too flashy that you can overlook a beat, maybe a dim option in percentage (like stepmania). And while you are at it, a light background option for darker skins.
I'm pretty happy with the default dimming. Its around 40% black applied during play, and most people seem to find this to be enough. If a background is too bright, it should probably be dealt with during the modding phase (or you can manually remove it if you really can't stand it).