These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Dude, Not Funny!: Putting gay men in strait jackets, though as with all tropes on this page, your mileage may vary. Some think it's just upright Unfortunate Implications, while some think it's a wonderful mockery of the sort of yuck gay people in many countries have to go through. It doesn't help that there's only one other reference to gay people in the entire seriesnote In Demons Don't Dream, Dug encounters the grouchy and unpleasant Fairy Nuff, who is annoyed that everyone assumes that, because he's a fairy, he must be gay. Dug solves his problem by telling him to spell his name "FAERIE" instead of "FAIRY".
Guilty Pleasure: Oh god yes. The puns, the tropes used, the plots... all combined combined with the Parental Bonus makes re-reading the Xanth novels 10-20 years out of the target audience a very guilty pleasure indeed.
That's if it doesn't produce a "I actually liked reading this? What was I thinking?" reaction.
Mary Sue: A fair few, but nowhere as bad as the three princesses introduced later - "Rhythm", "Harmony" and "Melody". Whereas characters previously had a limited talent, which was used in a clever way to solve a plot roadblock, the three princesses have the talent to (described in-text) literally "Do Anything" by using their ~ Summonable ~ instruments. Any inventive solutions to plot roadblocks were thrown out the figurative window, as the princesses (who showcase in most books) will solve the problem by themselves with no intervention by the main character of the story.
The moons of Ptero also count in a way, as a convenient dumping ground for pun libraries and easy character solutions, due to the presence of "alternate" plot characters and convenient havens for happy, age-inappropriate romances.
Nightmare Fuel: What Trent found in Castle Roogna's library in the first book.
Pandering to the Base: Xanth is read mostly by hormonal teenagers (of both genders), and Piers Anthony is distinctly aware of this — and includes a weird combination of fanservice and strong female characters for both sides.
Funnily enough, Anthony stated in one of his Author's Notes that the reason the Adult Conspiracy was hiding less in later books was that he was offended that his books were being put in the Teens section of the library and wanted to emphasize that the books were meant for adults.
Except the novel is based off of the game that is based off the novel. It's maddeningly recursive on Your Head A Splode levels. Basically, in the novel, the protagonists are playing the game that is now out, based on the novel, making it a bit of I Wish It Were Real, as well.
The Author's Note straightens this out: Anthony wanted to make a Xanth computer game, but lacked the expertise to do it himself. He wrote the novel to show Legend Entertainment what the Xanth game should be like, and they went and made it for him.
Running the Asylum, Ascended Fanon: Readers send Piers Anthony enough suggestions in fan mail that Xanth novels now consist almost entirely of such material. Piers Anthony acknowledges these contributions in his chapter-long Author's Note at the end of every book.
The way the Dolph/Nada Naga/Electra love triangle was resolved (which hinges on the Exact Words that the Good Magician Humphrey said) was suggested by a fan; in the Author's Note for Isle of View, Piers Anthony said that he hadn't thought of that solution, and had even already written an ending in which the wrong girl "won".
Sequelitis: And how. The first book was amazingly original, but by Harpy Thyme the prose is so badly written it can be painful to read in places, among other problems. Many people recommend stopping by Question Quest, and giving the later books a miss except those with characters from the early series (like the aforementioned Harpy Thyme, sadly).