It's merely my opinion, but I think that the label part is only important in order to target queries if you're seeking an agent. The agent will shop it (with approp slant) to the places it best fits. The house will determine where, ultimately, it best fits. Marketing, sales, publicity, & individual stores & libraries will determine the final labels & shelving. So my thought it to write the story, & worry abt labels later. Until you get to the shopping part, it doesn't matter what you use to label it.
And sometimes there isn't a single answer to this. N Gaiman's THE GRAVEYARD BOOK has a British children's cover, British adult cover, signed limited ed cover, & a US cover (
http://www.thegraveyardbook.com/bloomsbury-editions/) Is it an adult or kids or YA book? Yes. My own book will have adult editions in two countries. The content of the book is unchanged. The publishers make these decisions.
Arguably a lot of fantasy (as well as classics & general fiction) could be packaged as YA if it were printed today. My teen daughter shops in the SFF section & fiction & YA sections. The labels affixed to the books are for shopping & marketing, not necessarily reflective of who should read them. If the publisher is know for SFF & feels that the target market is not mainstream, the presentation will be different than if they feel that it's YA or crossover. I have one cover that screams SFF, but the book was shelved as YA. (It didn't do well in that country.) My adult edition covers are versions of my US YA edition. They aren't targeting just SFF readers.
And as to the lack of lengthy descriptive passages, I can think of a few YA books (& MG ones) that were quite popular & violate that idea all over the place.
The closest things to dividing lines I see btw YA & adult SFF are
1) possibly less sex--although Pratchett's adult fantasy books are sans sex and they aren't labeled YA
2) possibly the age of the characters--at least 1 protag is typically teen
3) possibly less meandering description--but a few of the biggest sellers in the US market the past years violate this to extremes
4) hopeful ending*
*Hopeful endings are seemingly more typical in YA across the board. Romance & mystery do often have hopeful ending (the HEA ending once was required in romance & some readers still think it is required). Mystery has resolution, so that's kinda hopeful. General fiction, horror, SFF do not require--and often do not have--hopeful closings. YA does more often than not.