I'm Jarno, I'm a 26 year old from Belgium. I got into reading fanfiction in high school and now as a 26 year old who's working as a product manager, I still enjoy fanfiction.
Couple things I don't understand in canon.
The fidelius: We know that the secret keeper of a fidelius can be in the house under the fidelius. (Dumbedore and grimmauld place.) So why exactly did they make pettigrew the secret keeper. Why not make James or Lily the secret keeper? Now that way they would actually be safe. I think Rowling should have written more about such an important piece of magic instead of just skimming over it.
Harry's cloak being a hallow. Moody, Dumbledore, the Marauder's map can all see through the cloak. So it doesn't exactly fit the deathly hallow description.
Harry ending Voldemort's life with the unbeatable wand, more like malfunctioning wand. So what happened to the power the dark lord knows not? How did love affect that wand in anyway? I found that a deus ex machina solution.
Malfoy going free and Harry atcually agreeing with it. Sure he didn't kill anyone, but only because he failed to do so. But attempted murder is still a crime, isn't it? Malfoy also had no problems with using the imperius, cruciatus, poison, etc.
Harry/any uncommon pairing
and before HBP: Harry/Ginny. HBP: "You're only happy if you're out there hunting Voldemort and that's why I like you so much." That one sentence put me off from H/G. I still do quite enjoy a well written H/G ship sometimes.
Ships I can't stand
Ron/Hermione: It's a ship I simply do not understand. I find their core values as well as core fears to confict to much, and for me it's a potential abusive relationship in the making.
Harry/Severus: that's just sick. Severus is literally as old as Harry's father, this is pedophilia.
Plots I don't like:
Hermione betraying Harry. Hermione and Harry are by far my favourite characters so I'd like them to at least be friends. And I don't like either being bashed.
Dumb Dumbledore. You often see this in manipulative Dumbledore fics where Dumbledore acts like an idiot. He is not, make him manipulative all you want but he is brilliant. Besides what's the fun in reading about a dumb manipulative Dumbledore, give Harry a challenge in overcoming Dumbledore's manipulations.
Characters I don't like:
Ron: He has his good qualities but he also has major flaws and for me his flaws outweigh his good aspects. The way he treats the first years is plain bullying. Outside of that he expects glory to just fall in his lap, is often prone to jealousy and violence.
Snape: He should have been kicked out of the school a long time ago. If there was one occasion that Harry should should have used his fame, it should have been to get Snape fired.
Malfoy: Especially the younger.