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 from Midnight Castle, period.
Ear Worm: a good chunk of the songs are very catching and will stay with you long, long after you've watched these. In particular the opening themes.
Shoo be doo shoo shoo be doo!
Ensemble Darkhorse: Firefly. Spunky tomboy with guts and attitude? 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.
Catrina almost crosses this when she attempts to kill a Bushwoolie and Baby Moondancer. Rep saves the former, and regarding the latter, he stops her and makes her change her ways just in time.
Sacred Cow: Midnight Castle is this to G1 fans without doubt. Catrina to a lesser extent.
Seinfeld Is Unfunny: To a lot of FiM fans. Though these specials were pretty edgy for My Little Pony at the time, the current incarnation has taken everything these specials were and made them the standard for the franchise.
Shipping: With the advent of Friendship is Magic, shipping and Ho Yay has become a popular phenomenon in the pony fandom. So pairings like Firefly and Medley, Bowtie and Applejack, Megan and Scorpan, Twilight and Moondancer, and of course Catrina and Rep have begun popping up.
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, you may be less than impressed with anything that followed after.
Perhaps a bit unfair. Though it would take until G3 for the animation to get back to the original's quality, many of the series villains are quite menacing and the likes of Grogar and Lavan could give Tirek a run for his money.
True Art Is Angsty: Tends to be considered 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.