As a YA fantasy writer, I read lots of unpublished YA fantasy. I love it. I love reading things before they are published. I also read a lot of published fantasy. One thing I've noticed, in an unpublished book I read recently (don't worry, it's no one who reads this blog) and in a published work I read recently is that in both cases each author felt the need to explain things about their world before they were relevant to the story or made sense. The only thing it did was left me confused and I didn't remember half of what was said because I was still trying to ground myself in the story. I think this can apply to non-fantasy works too, but this is the advice I gave the unpublished writer that sometimes I need to remember myself as well.
"Trust in your world and trust that I will learn about your world as things come to pass. Don’t feel like you have to tell me every single thing about it. Part of the fun of being introduced to a new world is the ownership I (the reader) feel in it as I “discover” things about it on my own. So, pretend as though everything in your world is completely normal then you will resist the urge to explain everything."
Same applies to characters. Don’t tell me who they are, just let them be who they are. It will be obvious that they are stubborn or flirty or quick witted by how they act.
I know you’ve heard it all before, but sometimes hearing it a different way is helpful.