Over at SF Signal, various authors and fans have offered opinions on which is better in a work of fantasy or science fiction, polytheism or monotheism? The best answer (to my mind) comes from John C. Wright, whose lengthy essay on the subject can be summarized as, it depends on what you're trying to do and how you think you can accomplish it. Sounds sensible. Kate Elliott adds another sensible answer, which is that the religion, whatever it is, should be thoroughly worked out as a part of good world-building, and should interpenetrate with the other parts of the culture.
To my own mind, even if a writer doesn't want to go to the trouble of carefully working out a theological system, he should at least be good at faking it: I've once or twice complained about the stripped-down religious systems in some Evangelical fantasy works; I suspect the religions in such novels look so basic and dull because Evangelicals, as a rule don't appreciate that long-lived religions accumulate ornamentation. If an author doesn't want to bother building his religion from the ground up, he should at least know how to thrown in the ornaments; you can hide a lot of vagueness behind a fog of incense. Of course, this is a bad idea when the religion is front and center rather than part of the background decoration.
But I do not see anyone in this discussion making much mention of pantheism (where everything is god), henotheism (where you acknowledge all gods but worship one), or, for that matter, atheism.
Now that I think about it, I can't off-hand think of any examples of henotheism in fantasy fiction except Orson Scott Card's Hart's Hope, which has a god called God whose followers worship him exclusively, even though the other deities, the Hart and the Sweet Sisters, unquestionably exist. For some time, I've had percolating in the back of my brain an idea for a post-singularity sf story centered around a henotheism of sorts; maybe I should work on that.