Here's an example of what Katz was talking about, this is my brother-in-law's track. The unmastered version is on top, the mastered on the bottom. I've removed the name just so that no-one gets any crap for it (like me),
http://img44.imageshack.us/img44/5967/waveform.jpg
Now granted the mastered version isn't a disaster or anything and most people won't notice much of a difference, but the first one sounds more open and more like a performance to me on my system...and I'm sure I'll notice it even more once my main listening room gets treated acoustically.