What are the Cardinal Sins of DMship that should be forbidden?
1. Geas/Quest abuse.
I HATE that. Sheesh, I'm not sure Geas should have ever even been written. I think it can be used responsibly, but that is difficult to do. Any time you are forcing the characters to follow a course of action because they will probably feel kinda crappy if they don't, ... you need to reevaluate what you are trying to accomplish as a DM.
2. Swiping a spellbook.
It's just ... mean. The player had really better be an a$$hole if you are going to do that. Not to be confused with the character being and a$$.
Ya know what? The players may NOT want to do that quest! So don't force 'em. It sucks, yes. Spent a lot of time on it, I bet you did. But, get over it. It's all about the roleplaying, and metagaming characters into a quest they wouldn't go on just sabatoges RP.
Having said that, one of the reasons I recognize all these is because I've committed them all somewhere along the way. As a new DM, I saw Geas and said "OOOOH!" and of course (ab)used it. Wups. I also was guilty of the second two in the same campaign v.v (which admittedly was my first attempt at DMing. Sorry emagius
! I'm sure you are remembering baaaaaaad, bad sessions right about now. ~_^ Heh, thanks for enduring my newbien DMness. I'm much better now, I promise. ~_^) I'm lucky I've had such great players!