First, third, omniscient, epistolary -- we even hit on second-person once. The panel did not approve. I recounted the one instance I had run across and found it effective: a woman recounting the story of her sister's life to the clone of her sister while she starting the clone developing. Another panelist thought that sort of situation might work.
But we stuck to the classical ones, which work.
The importance of choosing your POV character. I brought up Holmes, and we thrashed about the reasons why the stories where he's the POV character are so much worse than the Watson ones. You'd have to know everything he knows, you'd have to see the moments of hesitation -- it's so much more dramatic if he stands in silence and then picks out the obviously right route, but in his head, he has to go through it.
How multiple viewpoints trade off dramatic irony -- revealing things your characters don't know to you -- versus identification, since you are not in one character's POV.
The need to not allow POV characters who keep secrets. Unless you're going by unreliable narrators, in which case, you need to have the narrator presented as telling the tale in some way, so we are alerted to his ability to withhold facts. If you do the first-person is third-person with different pronouns, and give us the camera's eye into his head, secrets are cheating. (Also in third-person -- if we are getting the character's thoughts, we're entitled to them straight.) One panelist wanted to talk about The Murder of Roger Ackroyd -- but that would spoil it, and most of us haven't read it. I brought up Till We Have Faces, instead, because there we are told on the first page that the narrator is writing a book -- this book.
The vast important of making POV shifts clear. Along with accompanying place and time changes. Sticking the name, place, and time up top on the section can do the trick.
One audience member wanted advice on whether to use third or first. I advised that first person lets you do more tricks with voice, but then, it focuses attention on the voice. Others advised that if you are more interested in the tale they are telling, first person; the action, third person.
Massive number of POVs got severely criticized. Particularly those where the connections are not obvious. I pointed out that in The Lord of the Rings, except for the very first, you never get a POV character who was not first seen by another POV character.
One audience member asked for advice about using third mixed with first. She was warned that you really, really, really need to do it more than once. To establish a pattern. Once looks like you are just doing it because you couldn't keep to the POV structure. (Unless you do it in an epilogue or prologue.) I mentioned Ciaphas Cain as the best example of such mixing --- but warned that it was really epistolary, so perhaps not useful as an example.