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.
Angst? What Angst?: Despite the fact every character was at Rock Bottom before coming to Castle Perilous, and several were contemplating suicide or had almost died, this is never really Played for Drama or even focused on much, usually revealed as an offhand aside. The exceptions might be Sheila's past, which she spends some time discussing with Trent, and Linda, whose psyche and loneliness are both explored in Castle Dreams and Castle Spellbound, but even these are not dwelt on as much as they could be. In some cases the characters seem to have Epiphany Therapy and get over their 'angst' but in most it merely gets left by the wayside.
Big Lipped Alligator Moment: The whole series is known for these, since it's predicated on the idea that you could find anything, absolutely anything, through the next door or around the corner, which has nothing to do with the rest of the castle (or story), such as Melanie's adventures with the Robin Hood Expy. But the entirety ofCastle Spellboundis this, since the wish spell which takes over the castle comes out of nowhere, has nothing to do with the rest of the series, serves no purpose except for wacky hijinks, and once banished is never mentioned again. And since the cross between All Just a Dream and Literary Agent Hypothesis means none of the events of Castle Dreams actually happened, it never gets mentioned again either.
Designated Villain: Ferne—for certain in Castle Kidnapped, but even in Castle for Rent she only wishes to be granted a third of the ruling power along with Incarnadine and Deems; she is ruthless, willing to do whatever it takes to gain what she wants, and made the mistake of allying with the Hosts of Hell, but she never meant the castle, its residents, or her brother any lasting harm so long as she got the power she coveted.
Everyone Is Jesus in Purgatory: Every Guest who ever comes to the castle does so when they are at Rock Bottom, no choices left to them, utterly depressed, Driven to Suicide, or even in a life-or-death situation. A case could be made for an alternate interpretation where they really have all killed themselves or died, and the entirety of Castle Perilous and their adventures there is a Dying Dream, or a fantastical version of the Afterlife. Melanie actually raises this possibility not long after she comes to the castle.
God-Mode Sue: Linda. The only thing that keeps her powers from taking over the story is that sometimes they do fail, or bring the wrong thing, and that quite often the solution to the current issue facing the castle is something she can't conjure. But she usually is a very big part of saving the day nevertheless. Unless it's the other resident sorceress, who has a Story-Breaker Power.
This claim could also be made of Incarnadine—at various times he is actually accused of godhood, or responds to the question by tongue-in-cheekly claiming to be a demiurge, and he certainly is, to use Osmirik's in-story Understatement, "something of a magician". In fact not only is he capable of things no one else is (creating and building computers by magic, creating technology with magic that lasts longer than anyone else's, understanding and applying numerous arcane systems, making three-dimensional spell components (and drawing complex two-dimensional ones by hand with perfectly accurate lines and angles without aid of draftsman's tools), memorizing complex spells after only reading them once, and so on), but he is usually the only one who fully understands what is going on and can save The Multiverse. The end result is that whenever a crisis ensues, he must inevitably be absent, trapped, or deliberately removed from the castle so that there can be any kind of narrative tension. Sometimes there is more than one issue at stake, so that he can handle the one on The Multiverse level while the other characters deal with the more "mundane" problems.
Post Script Season: Bride of the Castle very much feels like this. Aside from the somewhat lengthy publishing gap, Castle Spellbound had ended with Gene and Linda declaring their love (and consummating it), then putting plans in order for their wedding; all the characters showed up together in good condition after the latest disaster; and the final paragraph of the book actually played Happily Ever After straight. Flash-forward to the next book where the Idiosyncratic Chapter Naming has been dispensed with, new characters are introduced whose stories have nothing to do with the rest of the book or characters (and one, due to being from an Alternate Universe, never will), the seeming happiness of Castle Spellbound devolves into the same old The Casanova behavior from Gene to make sure Status Quo Is God, and everything (but particularly the newest wrinkle in the Love Triangle) is left up in the air. With all these changes and unsatisfying elements, the fact DeChancie seemed to have stopped caring, and the fact that eighteen years later there has still been no resolution to the Cliff Hanger/No Ending, fans could be forgiven for wishing the series had ended with Castle Spellbound.
Sequelitis: Averted at first, for even with the Recycled Premises all the books up through Castle Murders are just as good as the first. Castle Dreams, being so surreal, isn't for everyone, but is still quite good. After that things go downhill a bit...
Technology Marches On: The scene with Linda and Melanie on the Internet (or Cybernet) at the start of Castle Murders is quite clearly dated; not only does Melanie express shock at the idea that a reply to her on a bulletin board could appear instantly instead of taking days or weeks (even suggesting the only way Linda could have replied instantly was if she was on another computer on campus), and specifically notes it is not a direct-communication system, but the ability to send messages directly to Melanie's computer for a real-time chat is portrayed as something only Jeremy's magical hacking can accomplish. In DeChancie's defense, the book was written in the 90s when the Internet (or at least, its modern, public version) was first in its infancy, but it still reads as amusingly quaint.
They Wasted a Perfectly Good Plot: This could be considered to apply to the series as a whole, since despite the countless worlds, characters, and plots available, DeChancie only focuses on a small (almost completely human) group and similar plots; even taking into account he couldn't possibly cover an infinite variety of worlds, characters, and plots, a lot more could have been done with the concept. A specific example, however, would be the plot of Castle War!—after raising the possibility of invasion by a Mirror Universe, and in fact having both Evil!Incarnadine and his guardsmen and Evil!Linda raise hell in the castle, the plot gets abandoned: the multiple Incarnadines end up being Played for Laughs instead, and after Linda's apparent death and Evil!Gene going off with Evil!Linda, the story focuses exclusively on the real Gene's adventures, Incarnadine's attempts to get home and fix The Multiverse, Jeremy and Isis's subplot, and Thaxton and Dalton's travails. When it finally returns to the matter of the Evil Twins, the solution is to Hand Wave it by having Linda and Sheila create an animated flying chainsaw to chase everyone back into their own Aspects, and that's that. The problem is dealt with by the spell Jeremy and Incarnadine cast, but it still seems a bit anticlimactic.