Since it seems like such a dead thread posting wise (though quite alive in the comments!) I thought I would start a new little discussion. What is your chief failing as a GM? Maybe it is something small, or such an awful mistake that your gaming table stopped playing and you lost a whole group of friends. Hopefully not, but bad stories are usually more interesting...
I know in the campaigns I have run, my biggest failing is the lack of role playing. I can write up worlds, campaigns, regiments, or characters, and have no problem playing as them. However, I suck at compelling my players to role play too. Part of this problem is my whole group's recent entry into the role playing world, with some of the players less imaginative than others. Regardless of that fact, I am bad at getting them to role play along with me. This isn't always the case - we had a very good Only Necromunda campaign everyone participated in nicely - but it is usually the way the cookie crumbles.
How about you guys?