<<Probably not a lot once you have the experience to know what you want, how to get it, and have lots of objectivity.>>
i.e. a mastering engineer? Out of necessity, I sometimes "master" things in house. But wherever possible, I send them out for just the above reasons.
I've spent the past six years being a full time fader jockey, and feel like the sun is just dawning. And yes, like previously stated every project gets better. But the projects we work on are going into a very competitive market, and every ounce of professionalism counts.
I've been sending our larger projects to NYC to master at a "name" facility this past year. We had grown a bit discouraged with local mastering and were feeling the same sort of "we can do this just as well" emotions. And in a lot of instances, yes, we could. Then we started getting projects back from NYC. Yes, there is a huge difference.
As tracking and mixing engineers claiming that talented mastering engineers don't bring anything to the table that we couldn't do without some practice seems a bit shortsighted.
The eq, the compression, yeah, maybe. But think about mixing, how hard you hit gear in the studio for just the right IM distortion so the harmonics stack just so so the vocal hits the comp just so so you can't lose it on an Aura(awful)tone. You know, the Jedi aspects of the job that you can't vocalize other than "watch, and listen" to assistants, that you had to learn through thousands of hours of assisting and watching and then hundreds of mixes to get it through your skull... The window of brilliance that every single piece of equipment is capable of...
Does this mean engineers are incapable of doing both? Absolutely not. Rare and talented exceptions to every rule exist, I'm simply aware I'm not one of them. I track and mix, and then I hand it off to someone who has a level of professionalism I respect, and the ears to match.
My longwinded .02
-rich