Fix the major bugs that are found. Test for both bugs and balance. I suppose these both could be filed under “listen to your testers.” It is fine to release live with some known minor bugs; there is a trade-off between delaying content and making a more perfect union. We know that we will need to fix some things after the patch.
I know that changing something by 20% is just a minor tweak to the code, but it makes a pretty big difference if you do so after the new content has been out for several weeks, and it looks bad when players were pointing the problem out on your test boards since the day it hit the test server.
Internal Q&A is probably very good. Few things make it to the test servers completely and utterly broken on any game. The goal is to see what happens when 10,000 people all bang away at it. Your internal team could not run through each part of the content 100 times, so this is desirable.
That should not be the only purpose of the test server. Yes, it is a small and unrepresentative sub-sample of your player base, but if someone is willing to put 10-20 hours a week into testing the new stuff, you might want to hear his conclusions about how it plays in the field.
You also might want to give players boosts of various sorts to let them test effectively. For example, if your players need to spend 50+ hours leveling characters on the test server to test the endgame content, your endgame content is going to be tested pretty badly. Perhaps this is why non-bug report feedback is often set aside: two wrong make a “meh it doesn’t matter.”
Say you release content and balance it upwards later, by which I mean buffing the new class, skill, power set, whatever. Who will react more strongly: the people who are happy about getting a bonus to their new class, or the people who are really bitter about having ground out 20 levels with the gimped version? There will be posts thanking you, but what about the rather lengthy threads discussing the issue for preceding weeks? After a month on test and three thousand-reply threads about how the new class is too weak, it goes live as is, but gets changed two weeks later? Did something change in those two weeks, or are you trying to give players the impression that their test feedback is unimportant?
Personally, I no longer play on test servers. It is close enough to paying to beta on the live servers.