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.
Canon Sue: Oh dear, Emily. Her name is Sue-ish to start with, 'Byrd Starr'. Emily has Kaleidoscope Eyes, which can go from a suspiciously Sue-ish purple to grey depending on her mood. She has pointed 'fairy-like' ears, and 'cloudy' jet-black hair which we're often reminded is glossy. She has a tragic past (orphaned), and her guardians are frequently unreasonably harsh towards her. Just about every man who encounters her seems to fall in love with her - Dean, Perry, Teddy, the Japanese prince...the list goes on. Did I mention she has vague psychic powers? And out of all of LMM's characters, she's likely the one most closely based on Maud herself.
She has enough flaws in the novel to keep her from falling into Canon Sue territory. She can be cold and sarcastic and has a fair degree of the Murray pride, even snobbishness. She also gradually realizes that she has exaggerated Aunt Elizabeth's hardness. As for her looks, she is not extraordinarily beautiful—her high forehead is frequently mentioned and she seems to be almost plain unless it is hidden by bangs.
The show averts this - she's normal in appearance (though by no means ugly), and while she is shown to be more sympathetic than others, a lot of episodes show her as incredibly temperamental and childish. Her writing is also shown to be (quite realistically, given her age) flawed.
Nightmare Fuel: In Emily Climbs Emily is accidentally locked inside the church at night during a thunderstorm...with a madman who has mistaken her for his long dead bride.
Tear Jerker: Try getting through the first few chapters of the first book without your eyes stinging. And it doesn't stop there.
Values Dissonance: Notably appears in concordance with Cursed with Awesome: Emily occasionally shows psychic powers, which have been known to save lives and solve decades-old mysteries? Great! Except that she is ashamed and afraid of them and keeps them a secret, because of the stigma around insanity and all unexplainable phenomena in the Victorian era.
The Woobie: Several characters could qualify, including young Emily and possibly Dean, but Mrs. Kent deserves special mention as a Jerkass Woobie who is in desperate need of several forms of therapy, who deliberately wrecks the happiness of people around her, but Montgomery takes plenty of time out to say, she had it rough.