My take on this at the moment is as follows:

If your character has not done a quest, they shouldn't be talking about it as if they did. Do I expect you to pretend to discover the happy boot quest anew with each new character. . . not necessarily. Would I be grumbly if you said "Oh, well, you do this thing, and then this thing, and then this happens" if your character had never done the quest. . . probably. If you're playing a character who has never solved the Vivarium Puzzle, I don't think you should be telling others how to do it.

This the kind of thing I pulled Geramin aside to talk to him about. The Geramin character had been into the Tower, but had never really gone anywhere inside, done anything significant, or been through any of the little quests or puzzle solutions that he was talking about. I got tired of him telling me that I was making assumptions and that I couldn't prove anything, and I told him several times that I was quite certain and not making assumptions. I even went so far as to tell him to not say that (being that I couldn't prove anything) again. He said it again and I decided to stop wasting my time.

Beyond that, just try to put yourself in my (or whoever wrote the area's) shoes, and the shoes of the person you're telling things. I'd like to think you can balance not completely ruining an area and the fun of exploring it with not being a total ass.

Of course, I now tend to write quests and do things in areas that don't behave the same from character to character. Let he with previous knowledge beware.