Per-application volume control. Not the Tiger approach – you have one volume control for system alerts and one for everything else – but the Windows Vista approach, where you can set individual volume levels for different things. That way when some arse embeds audio in their site or Flash ad, it doesn’t play at the same ridiculous volume as the music you’ve been listening to.
Not a particularly interesting post, I’ll admit. But that little change would make the internet much more pleasant.