Angst? What Angst?: The Mode series was basically the first time that Anthony ever depicted a woman as having actually been traumatized by being raped (and apparently that was only because he'd recently become aware of the practice of "cutting" among teens and wanted to put it in a book). Every other female that has been raped in his writing generally responds to the experience with the emotional equivalent of "Meh, whatever."
Protection From Editors: In his Author's Notes, Piers Anthony has described several bad experiences he's had with editors early in his career and says that, now that he's become successful, he no longer allows anyone to make changes to his books.
The Apprentice Adept series starts off on a colony world named Proton where only the members of the ruling class (called Citizens) are allowed the privilege of wearing clothing. The majority of the population consists of their indentured servants, called "serfs", who are required by law to go naked at all times.[1]
The flipside to this is that, while full nudity is no big deal, partialnudity is an instant turn-on. One of the protagonists on said naked world, for example, asks his lover to (illegally) put on a chemise and immediately notices how much sexier and more interesting her body becomes when he can't see all of it.
Sequelitis: The Xanth series is up to 35 books as of this writing.
Unfortunate Implications: Most parents would be rather shocked to discover that the risqué situations and jokes in Xanth are both understood and appreciated by their children.
The nature of Anthony's correspondence with a rather young girl, which he compiled and published in Letters to Jenny, occasionally borders on unfortunate implications or, depending on your view, outright Squick as he'll often do things like ranting about social mores regarding the innocence of children or discuss various details of the porn (or comedy, whatever) novel mentioned on the main page. (In fact, he specifically points out the scene he wrote in Pornucopia of supposedly consensual sex between a man and a five-year-old girl.)
"As a kid, Piers Anthony's books read as juvenile crap for weak minds. As an adult, they read like an [sic] search warrant for the author's house." -Burgonet, in 'To: Piers Anthony. Re: Getting therapy'
Even shorter version: Piers Anthony is a pedophile.
↑There are exceptions for safety gear and such, to cover the corner cases.