In your opinion, what makes a YA fantasy novel boring?

Too much worldbuilding.

I know, I know. I'm probably in the minority here. But when there are too many foreign names/creatures/etc., I find myself wanting to close the book and run. Maybe that just means high fantasy isn't for me, or maybe it means that less is generally just more.

Well, remember, there's a difference between worldbuilding - which is the art and practice of building worlds (duh!) - and exposition based around that building.

I believe there is never too much worldbuilding. As an author, you HAVE TO KNOW these things about the place you're writing. Every step you take to make it feel and be belivable the better. However, the reader doesn't need (and in fact, shouldn't have) everything jackhammered into their head by the exposition fairie.

I have a question for the OP - what could possibly apply to YA only that is not true for all genres?

What makes a book boring? The can be as long as there are readers' preferences or fairly short:

• boring, flat, uninspired writing
• no / weak plot
• weak characterization
• unsympathetic characters
• insufficient development
• and last, but not least, it just doesn't grab that particular reader

Agreed. You need to know where all the chairs are and the way to the kitchen, but your reader doesn't. If you've built the world or place right in your head ( or as in the current WIP, you are using the model castle your partner built), you will convey that feeling of being there without over selling it.

-Deus ex machina.
-Never getting the feeling that the main characters are in danger (or go through character growth, for that matter)
-Being written down to (you know, the simplistic writing that treats you like you need your hand held, that leads to you knowing almost exactly how the plot is going to play out after reading the opening paragraph)
-Terrible dialogue. If I wanted to deal with something that sounds like someone's weekend D&D sessions, then I'd just play AnD. I have literally read (part of) a book that I couldn't help but imagine players sitting around a table.

I dislike when there is this awesome magic system, but the protagonist becomes this omniscient God type character at the end anyway. With villains, it's less annoying because that's part of the trope with their powers sometimes.

I totally agree that sex shouldn't be included (especially when it's explicit--implicit is a lot more acceptable. It's just... I've read some YA books that were basically erotica and that makes me so uncomfortable). I don't mind romance, though! I used to hate it, but now I like it a lot, as long as it's secondary to the plot (and as long as it doesn't contain too many boring hetero tropes... like a guy swooping in to rescue a girl, etc. Or obvious, try-hard subversions of hetero tropes, like a girl swooping in to save a guy). Names get me, too. If they're too silly, I put the book down. (If they're based on something that's relevant to the story, though, I don't have an issue. Like, my characters' names occasionally look a little odd, but it's because they're drawn from a foreign [and also dead] language.)

The WORST thing is when the author totally misunderstands medieval European society, but still attempted to write it. Maybe it's because I adore medieval history, so the errors are more obvious to me... but I hate it so, so much. It physically pains me when someone clearly did not even attempt to research. (Honestly, an obvious lack of research in general is awful.)

I despise, despise love triangles and a romantic subplot that takes over the entire plot. A lot of YA novels do that and I just want to throw the book down. Like, hello, your kingdom is going up in flames and people are dying and the Dark Lord is going to take over and you're apparently the only one who can stop it but you're going to parties and balls and fussing over your makeup and two guys who are in love with you?!?!​ Where are the priorities?!

Edit: I just realized your post asked for elements that make a YA boring. Well, you could say the above would also bore me enough that I'll DNF it. Oh, and bad writing. And cardboard-cutout characters.