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.
Big Lipped Alligator Moment - David's son playing with the monkey, wearing googly eye glasses and singing "Rock and Roll Martian" over and over. As noted in the Mystery Science Theater 3000 episode guide, all the writers were just fascinated by this scene: "It seems like such a pure 'kid' moment; how did they ever get it on film?"
Designated Hero: Merlin is portrayed as the good guy, even though he tricks a man into summoning a demon and burning down his house.
Mike: Clearly Merlin has brought good into the world!
Oh, it's even worse when you think about it. Merlin wants to bring magic back into the world - so everyone can do things like summon demons and wreak havoc, or at the very least age horribly quickly from using it.
Well, that guy WAS a jerk and he can't control the magic, which is why it gone horribly wrong. Merlin might have intended to bring good magic to children and basically good people.
Funny Moments - This exchange between Grandpa Borgnine and his grandson:
Child: The monkey killed the dog? Borgnine: I told you, it's a bad toy. Child: What happens next? Does it kill Michael? Borgnine: Well, now you're getting ahead of me.
Glurge - So, you have Ernest Borgnine telling stories to his adorable grandson about funny old Merlin just wants to bring magic back to the world, as he tells the wide-eyed kid watching the puppets. Then the lady who can't have kids comes in and gets all teary-eyed as she tells random strangers about her husband's infertility issues. THEN Merlin loans his book to the mean old husband, to teach him a lesson about believing in magic. Said husband then burns a cat to death with his fire breath. Hooray!
Hilarious in Hindsight: The rise of sites like Yelp has made giving a store a bad review a pretty effective threat.
Special Effect Failure - The first half of the film has horrible special effects. In particular, when Johnathan starts breathing fire, the source of the flames is clearly several inches away from his face. Johnathan's aging makeup is laughably bad as well (he has an open collar and they forgot to apply makeup below his neckline!).
Mike Nelson: You know, Satan could have at least blended the edges of his bald wig a little better...
What Do You Mean, It's for Kids?: Merlin makes the exact same mistake that Pod People did: shoehorning a crappy horror movie into the package of a crappy kid's fantasy movie and then forgetting to take out the "horror" part.