I think a lot of what I dislike in RPGs arises from what I call "Frustrated Novelist Syndrome". This psychological disorder affects approximately 67.8% of geeks, according to my impeccably-conducted empirical research, and has a definition in the DSM-IV as "Wanting to be a novelist very badly indeed - so much so that it manifests itself in ones gaming in all manner of undesirable ways".
Frustrated Novelist Syndrome (FNS) is primarily responsible for a heck of a lot of game-manual fiction, which is without doubt the worst sin committed by game designers anywhere, in any context. (By definition, if you were a fiction writer worthy of being published, you would be being published as a fiction writer, not as a game designer.)
But it is also, I believe, responsible for the worst sin committed by GMs in general: railroading. Having a 'plot' which the players are supposed to follow in a more-or-less predetermined path, fudging a large percentage of dice rolls, and assigning missions and goals to the players from the beginning - all of these make for less interesting gaming in my view, and they are all problems that arise deep within the psyche of a GM who really wants to tell a story of his own devising. It is the frustrated novelist inside him wanting to get out. It is FNS manifesting itself in destructive and foolish behaviours.
The only known cure for FNS is random generators, and lots of them, combined with burning the sufferer's DM screen so dice fudging becomes impossible. Another, more experimental, cure, is being conducted under the auspices of the Indie Game/Story Game movement - with the aim of removing narrative power from the GM. Results so far appear to be mixed.