Aluminium Christmas Trees: The concept of a person's physical health deteriorating due to being away from home may seem strange to modern audiences, but until the late 19th century, nostalgianote Originally a Greek word meaning "pain for home"; the English word "homesickness" is derived from it was considered a genuine medical condition. It was first observed among Swiss mercenaries serving abroad, and some deaths from it were even reported. Severe homesickness could actually receive a "schizophrenia, undifferentiated type" diagnosis as late as the 1980s.
First Installment Wins: By virtue of the two sequels, "Heidi Grows Up" and "Heidi's Children," being written by her English translator Charles Tritten and not by Spyri herself.
Germans Love David Hasselhoff: Heidi is a classic in the German-language sphere as well as the world, but it is notably popular in Japan, Italy and Turkey.
Glurge: Enjoyable Glurge, but still. The book really hammers in the idea that being outside is good for you and being cooped up indoors isn't, and Klara learning to walk just because she spent time in the mountain air can be a bit glurgey to modern readers, since we now know that many disabilities just don't go away like that.
Strawman Has a Point: Fraulein Rottenmier has a good point to make concerning Heidi's manners and personality, but her actions come off as harsh and cruel. Klara and her grandmother at least are more patient with Heidi, even helping her how to read.
Come on, try not to go for the hankie whenever Heidi visits Peter's blind grandmother. Or whenever she isn't able to visit her.
Come on, try not to go for the hankie when Klara finally leaves her wheelchair.
Or Heidi's Heroic BSoD that leads her to become a sleepwalker because she cannot stand being away from home.
To a lesser extent but still applies, when it is hinted at Alm-uncle's backstory that he left the army not for a murder, as often rumoured, but for the death by illness of an officer he had under his care as a male nurse sort of way, and said death was the beginning of his breakdown. Consider now that the first time the readers have access to those memories is by association with Klara, and that by helping her recover, he appears to be paying tribute to a man he admired and could not help, and...
It's strongly implied that Heidi's sleepwalking spells are caused by a propensity to epilepsy — her Missing Mom Adelheid had seizures ("curious attacks, during which no one knew whether she was awake or sleeping"), and at the time the book was written epilepsy was thought to be an inherited trait. Modern science is less clear on the subject — there seem to be some inheritable risk factors, but it's usually not something that recurs from parent to child, and most cases of epilepsy have unknown causes.
Heidi's little book of parables about what happens to children who don't learn to read includes sending them away to live with the Hottentots.
While it was also heinous at the time it was written, many modern readers find Peter destroying Klara's wheelchair absolutely unforgivable, thanks to increased social awareness about disability rights.
The 1968 movie
Mainstream Obscurity: The 1968 TV movie is known to a lot of American people only because of the infamous "Heidi Game" preemption.
Nightmare Fuel: So, so much in many of the adaptations. Fraulein Rottenmeier is one of the egregious examples of this in a few of the cartoon versions, but especially in the Hanna-Barbera film Heidi's Song.note One of only three movies they ever made that didn't feature their trademark characters (the others being Charlotte's Web, and Once Upon a Forest. There's one part where she turns into a fanged witch and...well, you should really just watch it for yourself. Or don't, if you'd like to sleep again.
note was considered a genuine medical condition. It was first observed among Swiss mercenaries serving abroad, and some deaths from it were even reported. Severe homesickness could actually receive a "schizophrenia, undifferentiated type" diagnosis as late as the 1980s.