I've traced this bug down to database tweaks we made to improve the board's performance, so it wouldn't slow to a crawl during heavy use (like when we get slashdotted).
Problem synopsis:
If you log in successfully, read a story (or try to post or comment, or anything), you hit "front page" or any section page again, and it makes you log in again or gives you "permission denied" if you are trying to post...it's losing your user permissions. Makes it hell to try and post. Then, the problem suddenly fixes itself.
From here, on 3 different browsers, if you log in and immediately click "User Preferences" that problem will fix itself and you won't have to keep logging in. I have no idea why, but I just made 3 new user accounts from here, all had the bug in them, and all fixed themselves after going to "user preferences".
Obviously this is a big bug--but at least for now this is a workaround. I've set new users to display User Prefs on first login, and am testing to see if that will at least help new users. We are getting a large volume of email from users because of this problem, and I am trying desparately to keep up with them and still do my job (this board is only supposed to be 10% of my workload, and it is at 99% right now ;~)
Still no luck on why comments don't appear if you are not logged in.
Any other user comments on these bugs appreciated, especially to see if this workaround fix works for you!
ADMIN
(that's Dan F, who is getting very grouchy and stressed about these problems)