Mastering is a really important part of the music (as said before by just about everyone), I myself have spent several years in professional studios learning how to master music. If you have a song in particular that you think is well worth my time, I can master it for you.
Mastering 'can' be fun, but I guess it takes the right sort of person to want to master. I really enjoy it myself which is why I guess I've gotten good at it. Things like unmixed 24 track versions of bohemian rhapsody make me squee for several minutes non stop and then I get to work for the next 15hrs straight mastering it!
Also
Edened wrote:http://howtomakeelectronicmusic.com/how-to-master-a-song-in-fl-studio
If you don't use FL this is may be useless to you.
I didn't actually read all of this but from what I scanned of it, I'd suggest not following its advice for the most part. There are elements of truth in it but you really want to do most of your mastering before you export the song as a single stereo file (which is basically the first step in that guide).
Edit: Sorry read a bit more of that guide, still stand by what I said before, but for particular genre of music (after already mastering the individual tracks) following that guide could be helpful.