What makes you cringe?

I am, as well, attempting to write a novel.
And it is, in true Kong fashion, a fantasy.
But not like unicorns and magic and pixie dust.
I'm attempting to make it more tangible type fantasy.
Well, as tangible as fantasy can be I guess.
But, what I'm trying to find out is, what elements of fantasy fascinate the reader and what elements make the reader cringe?
Obviously, some types of fantasy have flourished (Harry Potter, LOTR), and some are crap.
What is it that makes one like fantasy?
Is it the setting? The characters? The fact that it takes place in a world different than our own?
Obviously, this is a question that can have many answers.
The main struggle I've been having right now, which affects pretty much the whole storyline, is how old my character should be. I initially planned on having him be a recent college graduate (obviously easier for me as the author to relate to) but have been having second thoughts on placing his age at about late middle-school or early high school.
So, I guess I'm trying to determine if this should be a young adult book or an book geared towards an adult audience.
Help!