I am not satisfied with the book I am currently reading. There are a number of reasons for this, but at this particular point in the plot, it's because I realized that much of the book reads like a series of random D&D encounters. The sense that things are happening but not going anywhere is, for me, a prime source of frustration in a book.
Announcing this fact to the household led to the conclusion that a significant number of authors in our generation who were weaned on D&D may betray this influence. One way this shows up in characters who all but wear their class as a nametag. Another is in the magic system. I enjoyed Sanderson's Mistborn, for instance, but the magic system seemed so clearly to be rooted in the idea of stat boosts that it hindered my ability to immerse.
At Taos we discussed the idea that if you're going to use magic in your book, it needs to have rules and limits. This I broadly agree with, at least if your protagonists have access to it, because otherwise it's difficult to challenge the characters. However, I don't think the reader benefits from knowing what those rules are.
To me, one of the reasons to include magic is as a means to introduce the numinous into the story world. If it doesn't contain ineffable mysteries, then it's not actually magic. You certainly can treat magic as just another piece of technology, with
known and predictable results, a tool the characters can use just like a
gun, but I will never not ask why? One of the problems with D&D is that systematization and predictability remove the mystery from magic, and with it most of the joy that is specific to magic (you can still feel the joy of vicarious destruction, of course, if fireball = grenade).
In my own writing I try to make magic weird and slightly scary, to maintain that sense that it cannot be entirely predicted, and to give it a bit of poetry, although writing that makes me feel like I'm being unbearably snobbish. I don't want to write game supplements.