I thought that this video might be of interest with regard to the thread. IMHO mastering is more than simply copying the mix as accurately as possible. Mastering engineers are audio consultants, providing their perspective on how mixes might be taken to the next level or if in fact they need no further enhancement. When you hire an ME you are also hiring them for their tastes and musical aesthetic. Of course they do not have the final word but should service the needs of their clients; but if you don't respect their opinions you shouldn't be hiring them in the first place.
"Mastering" doesn't do anything. Neither does "mixing." Mastering engineers do something. Mix engineers do something. What that something is, really really depends on the engineer.
There is no such thing as unmixed. A record is always mixed once it's recorded. The question is whether or not the mix sucks. But as long as all the audio stems come down to the designated number of channels for final playback (usually 2) - it's mixed. Mastering - same idea. If it functions for the medium it is intended and can thusly be replicated for that medium - it's mastered. Might not be mastered well though...
Could your sound be better? Yes? That's where engineers come in.