For me, young adult fiction differs from adult fiction in that the "voice" is different--more youth-driven. Not just the use of slang or "imitating" a teen, but seeing the world from a teen's eyes.
Teens often see, hear, and respond to life differently than adults. They can be moved more emotionally by certain events. A rejection from a boy they like, a bad haircut, or their best friend moving to a different school can turn their entire world upside down. I'm reminded how utterly despairing Bella became after Edward left town in Twilight.
Coming-of-age or rite-of-passage themes are common, which differentiates young adult from middle grade stories (where younger protagonists have adventures but ultimately remain "kids" at the end).
Other young adult themes involve broken families, peer pressure, and all those oh-so-important "Firsts" (first love, first cigarette, first time behind the wheel).
Intensity is a word that comes to mind when I think of my teen years (and those of my kids), and I try to carry that out in my writing.
Alexandra Sokoloff has a wonderful blog post on Writing YA-Themes