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.
Complete Monster: Tirek is a surprisingly vile creature from a show that's supposed to be a little girl's show. A demon of a centaur, Tirek was the ruler of the Midnight Castle, and planned to send Dream Valley into a state of eternal chaos. To do so, he used a sack containing magical spells in it to essentially torture and transform young ponies into his hideous, winged dragons which he could then use to pull his Chariot of Darkness. He's also amazingly violent. This is shown whenever his henchman Scorpan failed him. He would threaten to have Spike's head cut off if he ever screwed up again, which makes it even more disturbing in that Spike was a child, and therefore it meant that he had no limits to who he had to hurt in order to get his way.
Contested Sequel: To a lot of fans either everything that came after these two specials, or just anything after Rescue at Midnight Castle, period.
Ensemble Darkhorse: Firefly. Spunky tomboy with guts and attitude? Check. Inspired Rainbow Dash? Check. Being voiced by Sandy Duncan also doesn't hurt.
Mr Moochick: this character is fondly remembered by many fans of the series as one of its finer points. Being voiced by Tony Randall probably helped.
Applejack, too, considering she was only in "Midnight Castle" but later got a song and a few generations later became one of the main characters of Friendship is Magic.note Of course, this was in part due to legal reasons. Applejack is apparently the only pony from G1 that Hasbro kept the trademark for.
Tirek is also surprisingly popular with the Friendship Is Magic fans. The creators eventually brought him back in said series.
Medley, Bow-Tie, Glory, Twilight, Moondancer, and Ember all have garnered their place amongst fandom.
And of course Habbit, the Moochick's long-suffering rabbit assistant.
Gannon-Banned: Some fans will insist that it's Rescue at Midnight Castle not Rescue from Midnight Castle, since it was given this name in it's later incarnation as a regular series episode. However, this is an aversion as it was originally called simply "My Little Pony" when it was first aired on 1984.
Nightmare Fuel: Tirek, everything from his voicenote Thanks to the Victor Caroli, AKA the Transformers narrator. to what to turning ponies into a dragons, even leaving one out and forcing her to watch.
Sacred Cow: Midnight Castle is this to G1 fans without doubt. Catrina to a lesser extent.
The former was a veryEvil Overlord with intentions of ushering an age of eternal darkness and thus rule Ponyland and possibly Earth for eternity. To do it, he put the local enchanted creatures through painful transformations and constantly kicks his right hand goat-man, Scorpan around without relent and with threats of decapitating his only friend Spike. Scorpan himself was formerly a prince who had previously been enslaved by a previous conquest by this guy. He only got one half-hour to show his stuff, and he spent most of that time sitting on a throne.
Tough Act to Follow: If you start watching the G1 cartoons starting with these, depending on how you view these specials, or even the franchise as a whole, you may or may not be less than impressed with anything that followed after.
True Art Is Angsty: Tends to be considered by some fans among the best things put out by the My Little Pony franchise because of its darkness.
Viewer Gender Confusion: Applejack is still a girl in Midnight Castle, but the gravelly voice can make it hard to tell.
What Do You Mean, It's for Kids?: A mild example, but some who flipped onto the station to watch this couldn't have have expected to BEHOLD THE POWER OF DARKNESS.