5) The world is not adult enough. No, I don't mean the amount of gore or sex.
Unfortunately, some writers think gore and sex automatically equals credibility.
Take Raymond E. Feist. The "Riftwar Trilogy" was a very silly, derivative fantasy, peopled with Sues and Stus, but fun if you didn't mind switching your brain off. There were even some quite good plot twists, including a prophecy that turned out to be a hoax.
The later "Serpentwar" series? Just plain stupid. I mean, really, really, really stupid– basically just a rehash of the first trilogy, only now the threat's BIGGER and everyone's MORE POWERFUL– in other words, it's got Lame Sequel Syndrome. The most annoying thing of all is that the author is trying to disguise all this by introducing "gritty realism", i.e. sex, graphic violence and descriptions of bowel movements. This only serves to underline the fact that all the characters are cardboard cut-outs. Really pathetic.
Sorry, that was a rant in its own right.