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.
YMMV: The Erl-King
Foe Yay/Ho Yay/No Yay: The Erl King spends a lot of time whispering to the boy about how pretty it thinks he is. Which for some readers just adds to the squick factor.
Nightmare Fuel: Through its Maybe Magic, Maybe Mundane approach the poem successfully combines the Primal Fear of the child (something is out in the dark to get you) with the Adult Fear of the father (losing your child because you're not fast enough to get medical help). The end result is unsettling no matter how old the audience is.