One big question for mastering is how loud do you want your CD to be? That might seem like an easy one, "As loud as possible". Unfortunately there's no free lunch on this one. If you compare a CD mastered back in the 70s or 80s with one mastered today there can be a huge difference in level. Modern CD mastering is producing CDs that are very loud by utilising a great deal of limiting and sometimes crude distortion techniques like clipping. This produces albums that have no ebb & flow and can be very fatiquing to listen to. Don't forget that people own volume controls, they'll just turn it down then your record is quiet & squashed!
It can be quite informative to compare the original CDs of some old classic albums with the recently remastered versions of the same album,."Hotel California" for example has gone up in level by 9dBs. This is at the expense of dynamics and clarity, if you start off at 10 you've got nowhere to go (unlesss you've got something that goes to 11 of course). It's worth remembering that "Nevermind", for example, had no limiting at all. No-one said that was quiet when it came out.
Have you seen that Metallica fans are rebuiding their own version of "Death Magnetic" by ripping the songs from Guitar Hero, where they haven't been destroyed by limiting/clipping? Ted Jensen, who mastered it, has publicly distanced himself from it, saying it was like that when he got it & he'd never want to do that to music, & that's from Ted who is Mr Loud!
One common misconception is that a loud CD will sound better on the radio. Radio uses huge amounts of compression, and pushing more into those compressors acheives nothing. Most radio stations use processors made by Orban, such as the Optimod FM8400. An excerpt from its manual:
"There is a myth in the record industry that applying "radio style" processing to CDs in mastering will cause them to be louder on air or will reduce the audible effects of on-air processing. In fact the opposite is true: these CDs will not be louder on air, but they will be audibly distorted and unpleasant to listen to, lacking punch & clarity. We hope that the record industry will come to its senses when it hears the consequences of these practices on air"
you do have a choice