SineNomine
Minstrel
Aside from, as mentioned, the general tone of voice used from the perspective of the main character, I would've said "Length" in the past though obviously that barrier has been broken down somewhat. However, in general it remains true that YA books average much shorter lengths than adult novels. You're going to have a hard time selling a YA book over 80k or so words. You can definitely do it, but you may have to fight for it and accept that both agents and editors will want you to chop it down to that size if at all possible.