I'm not the world's most experienced panelist - yet - but I think I've picked up a few things.
I saw some complaints to tumblr that a panelist at SDCC didn't mention things that these people thought were important. I'm not going to go into details, but it caused me to think that a lot of fans...and new writers...don't know what the other side of the table is like. This is part of why I did the Public Speaking panel at Balticon, something I'm hoping to repeat at Farpoint in February.
Here's the thing.
A typical convention panel fills an hour of programming, but is timed to 50 minutes. The reason that is done is logistical. It gives time for audience turnover, for panelists who are scheduled back to back to move rooms if necessary, go to the bathroom, take throat lozenges, etc. That ten minute gap is really important - it stops the entire con from starting to drift over time.
The moderator's job is, in part, to keep the panel to 50 minutes. It's not a lot of time.
It's really not a lot of time. The Doctor Who panel at RavenCon nearly went over and I had to have time called on me the first time I moderated, which was the online reviews panel at that convention. (There's a bit of a knack to it and it often takes new moderators 2-3 attempts to get it right).
Because of this, there are two things con goers need to bear in mind:
1. It's not possible to cover every aspect of any issue in 50 minutes. I have stuff I wanted to cover in Public Speaking that I didn't get to, in part because I had a wonderful panelist who was sharing her extensive knowledge of Toastmasters, an organization which can be very helpful if you need to learn how to do it. But it's just not possible. Stuff will get left out. Questions will not get asked. There's nothing anyone can do about it.
2. Panelist etiquette, that new guests have drilled into them, is to answer the question you are asked and stay brief. Wandering off topic, speaking at too much length or, worse, digressing into attempts to sell your books? These things make a panelist look bad and run the risk of the panel hitting the wall and having to be cut off short, reducing time for audience questions. A panelist who doesn't address something that wasn't in the question is, in fact, doing it right. You answer the question you were asked, not the question you wish you were asked or want to answer. I was on a panel where there were some disability issues I badly wanted to bring up, but there simply was no opportunity.
So, please, don't judge panelists for not covering something. There's many good reasons an aspect of an issue might be left out - and 50 minutes just is no time at all.