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: The "Ooga Booga" number. It comes out of nowhere and doesn't do much besides confuse and annoy the three bystanders. In fact, the Grinch's laughter at the end suggests he did it for the sole purpose of amusing himself for about twenty seconds.
Foe Yay: "Will...will you please scare me some more, sir? I-I, I sort of like it." Well, Euchariah, that's certainly one way to stall the Grinch.
Nightmare Fuel: It goes without saying that the Paraphernalia Wagon sequences are this.
There's also the Slasher Smile the Grinch gives at the title card, perfectly setting the mood.
The second to last part of the ending. Sure, Euchariah managed to stop the Grinch from going to Whoville to terrorize the Whos, but the Grinch points out to the audience that "that wind will be coming back someday. I'll be coming back someday," followed by an Evil Laugh.
The sinister music accompanying this scene hardly helps.
Sequelitis: How the Grinch Stole Christmas is considered a classic; this...is not.