Professional CD mastering is first and foremost a decision-making process, and then of course it's about making key changes to the sound.
You can kind of look at a mastering engineer as a 'third ear' guy, an over and above producer if you will. The mastering engineer comes in right at the end of a project and needs to make key decisions, and suggestions on the overall sound and layout (song sequences, pauses, etc.) that will be presented on an album. Producers, audio engineers, A&R persons, labels and artists all trust the mastering engineer to ensure that they put out a top sounding album with maximum impact. For this reason a mastering engineer needs more than just experience in audio engineering, experience in other aspects of the industry is vital. A mastering engineer not only needs a good ear, but also a good feeling for what works and what doesn't, a good understanding and feel for different genres of music, a good grasp of how to make an album 'flow' from the first to last track, and much more.
Mastering requires a certain level of responsibility that very few inexperienced engineers can offer. The mastering engineer is after all the last person to 'judge' and make changes to a project that costs thousands of dollars. Many producers, audio engineers, etc. trust the mastering engineers' judgment, so it's easy to see how important this function can be.
A large part of the idea behind mastering is to give a project one final 'look over' by an experienced person who has not heard the material before (hence the 'third ear' idea). Usually by the time a mixing engineer or producer has completed mixing a project they have heard the songs so many times that it's really hard to look at it objectively. For this reason, (amongst others) mastering on major release albums are almost never done by the same person(s) who did the mixing or producing, although this practice is very common when it comes to the making of demo recordings or low key productions, mainly for budgetary reasons. In most cases the producer, audio engineer, etc. do not attend mastering sessions, they will trust the mastering engineer's decisions and suggestions. There are times however that they might attend a session because they have particular concerns, changes or edits in mind.
Mastering engineers need to have excellent relations with their customers, and should therefore not have a problem to discuss suggested changes with the people involved in the making of an album. Professional mastering engineers will tell their customer(s) right away if they feel there is something the matter with a particular mix or take. It sometimes happens that tracks are referred back to the producer or mixing engineer for changes, if you don't have an open relationship with that producer or engineer, you are not going to be received well.
Finally, mastering is not so much about what equipment is used as it is about the sound. If it sounds excellent, well balanced and has a good flow, why would it matter how you got there? Mastering is a responsibilty you owe your customer.