There's a fascinating discussion/debate/snowball fight/free-for-all over in the comments at Shaken & Stirred about the nature of YA (thanks to Gwenda for the link). For my own 2 cents--which I'm unwilling to add to the discussion itself because I hate confrontation--I wanted to quote from a review in the December issue of The Atlantic. It's a review of Nick Hornby's YA novel Slam, which, incidentally didn't get an entirely favorable write-up.
Favorable or not, the review did nicely boil down the positive traits that set YA writing apart while also making it worthwhile--the reviewer describes Hornby as "funny, empathetic, morally aware, [and] attuned to popular culture and lingo." The reviewer also didn't really seem to see "young adult" as a necessary category, which was refreshing, as was the presence of the review in the straight-up fiction reviews section.
There's also a cool web-only interview with Nick Hornby in case you're Nick-crazed like I am. In the interview, he said he wasn't specifically writing for a teen audience while he was writing the book. "It's certainly about a teen, and I would hope that teens would read it, but I'd also hope that if I had written the book about somebody who lived in Alaska, somebody other than Alaskans would read it as well." (That quote's for you, Colleen!!)
In the interview, Hornby goes so far as to say that he thinks most books written for adults are outright boring, so if you're looking for an antidote to all the anti-YA-mania, go check it out.