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.
Jumping the Shark - According to many reviews on Amazon, Koontz has done this quite a few times, namely with Darkest Evening of the Year, One Door Away From Heaven, Your Heart Belongs to Me, the last two Frankenstein books, and Breathless. Seriously, Breathless involves all of humanity's problem's being solved by the inexplicable appearance of adorable talking simians? What if we can't feed them all?
Moral Event Horizon - Koontz is willing to do whatever it takes to ensure that the readers absolutely despise the villain and everything the villain stands for.
Nightmare Fuel: The Retro Virus in Fear Nothing and Seize the Night, and what it does to the infected. Also the things that occur in the egg room in Fort Wyvern. Plenty of his villains and their actions are worthy of mention. Chances are if its a non-satirical Koontz story, parts of it venture into very scary territory.
Paranoia Fuel - The entire plot of False Memory, especially for anyone who has ever visited a therapist. The Big Bad is a psychiatrist who mind rapes his patients and forces them to do all kinds of things for his own entertainment. He implanted many of their phobias in the first place, and once he's done screwing with them (sometimes literally), he often drives them to creative suicides and/or homicides. He was doing this for twenty years before somebody finally figured it out.
Shocking Swerve: Unfortunately this is how many, many of the conflicts in his stories are resolved. When his heroes are facing impossible odds, he'll have an angel show up, or reveal that an autistic child has superpowers and teleported the villain away or can summon and control bats.
The original Frankenstein trilogy ended with one of these. Suddenly there was a super-powerful "reject" creation that grabbed Victor and solved everything!
Tear Jerker : Numerous moments can qualify, especially those involving apparent character deaths, flashbacks, or musings on the world at large.