Breaking news, right?
In high school I probably read a combined total of around a hundred million words of D&D-inspired fiction. That sort of thing should prevent me from ever becoming a real writer, but I think I learned something from the experience.
D&D novels, like most media tie-ins, are bad. Among other reasons, they're bad because they cannot use one of the most important tools in the speculative fiction writer's arsenal: the ability to tie the plot to the world's setting.
The best fantasy (like the best science fiction, which is one of the many sub-genres of fantasy that is fun because it's easier to believe it could be real) derives its plots from the rules of its world. The Lord of the Rings loses its power if you remove its underlying metaphysical and moral assumptions about the nature of loss, power, and the need to preserve the past. If The Lord of the Rings isn't conservative (in almost every sense of the word), it's nothing, and since the first wave of Tolkien clones hit in the 70s, critics have lamented the inability of Tolkien's imitators to understand that you can't separate Tolkien's plot from his own personal views.
D&D fiction goes further than that, creating an enforced bubble of artificial reality (the D&D rules) in which the plot must take place, but the D&D rules don't make for good metaphysics, for obvious reasons: a deep, metaphysically robust game system makes for a bad universal fantasy system. So D&D writers, who are not all terrible, substitute familiarity for the metaphysics-as-plot link that fantasy needs. Instead of building a story around a magic system (think Elantris; in fact, go ahead and think of almost any good speculative fiction), the writers parade before us one familiar item after another--here is a magic missile spell; here is a paladin; here is a beholder.
I'm writing this because, like with Lieutenant Worf, it's a lesson I learned too well, and I'm increasingly aware that my comic tips in the opposite direction: without a proper understanding of the metaphysics of The Water Phoenix King setting, the plot is incomprehensible from top to bottom. Tolkien, at least, had evil ghost-wizards trying to take over the world; by contrast, WPK's antagonists don't necessarily even look like antagonists until you understand the underlying rules of reality that motivate them.
It's interesting to see how many elements of my own writing--including things that I'm increasingly convinced aren't very good--are reactions, and overcompensations, to the terrible rubbish-entertainment I consumed as a kid.