A good master sounds fine. Alright, but does it has to sound well on itself or just relative to other tracks? Because music is relative, right?
And why do people have the intentions to prefer track A above track B, only due to the loudness of track A (in comparison with track B)?
Can the mixer do the master, or do you need an independent engineer?
Lots of question and I don't know the right answer. I think it also depends on other factors. For example the weather or your mood.
What I do know is.. that mastering a tricky thing is. And it can be a pain in the ass, sometimes. Because what you hear depends on multiple factors. I already mentioned two, but the most important is of course the PA / SoundSystem / headphones. What is the power of the system, what frequencies is the range, or better formulated reach?
What sounds will reach you and what feeling does it mean to you?
Every track will sound differently on a different sound system. And the experience of what you hear depends on what track you've heard before or afterwards. A good DJ is able to play a bad track sounding well. Is this true? DJ-ing is art. Do not underestimate this fact..
Mastering is art, too. Okay, maybe it is more a science. Let's call it engineering! It is some kind of physics what you learn(ed) at school. But this is way more fun! And sometimes a real pain in the ass.
In my opinion a good mastered track is a track that sounds fine on every system and important words are loudness but clear, full range but not muddy, dirty but bright but above all clean!
Think of a beautiful naked girl, standing in the snow on a top of the mountain asking you on a sunny day, wanna a beer?
If the answer is yes, then and only then you are a master. Forget about the sounds...
P.S. Nice wisdom from Buddha: If the student is ready, the master appears. #Buddha
Good luck with mastering!
BTW: I remastered my album (10 100) 11 and it is available on Beatport.