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.
Nightmare Fuel: Arguably the last episode, in which an asteroid strike nearly wipes out mankind.
Several of the Bad Ass creatures, not to mention the extinctions that wipe them out.
Special Effects Failure: One of the most common complaints people have against the series. The animal restorations are ugly as sin, and the graphics in general look as if they had been taken from a video game. The animals themselves come from a prehistoric Uncanny Valley. Walking with Dinosaurs this ain't.
Tear Jerker: Several of the creature's sad fates in the face of extinction definitely qualify.
One episode has one of the last two Tyrannosaurus rexes kill and eat the other, his brother, then it cuts to him some days later, trying desperately to stay on his feet, knowing if he falls, he's never getting up again. Anybody else flash back to the horse from The NeverEnding Story?