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!