A track has to be mixed well first before you can get a great master out of it.. Mixxing is more or less the sound levels .. while mastering is EQ'ng,Compressing,and more dynamic processing..so you see its a whole different feild..
There are several before and after "A/B" files on the MM site. Many of them were "problem" tracks to start with, but it might give you an idea of what can change (and perhaps what shouldn't).