YA doesn't mean books for teenagers or books that teenagers have the intellect to read so much as they are just books about kids ages 13-18, though some people prefer to go younger. If you're going by the standards of what a teenager could possibly read then everything is potentially YA depending on what teenager you ask and that's just nonsensical which is why that list above makes absolutely no sense. By the judgement expressed above every book is YA (from children's books to adult) and thus the label becomes entirely and utterly meaningless. Intended audience can be a factor but the line can just as easily become too blurry.
I'd say that novels about adolescents is part of the standard, but also novels targeted at adolescents as well. Both of those definitions fail to cover some classics that have been included in the YA catalog, though. The wikipedia article has a lot of interesting things to say about the history of the genre and how it's defined. I think for the sake of making a list, the novel should be widely recognized on some level as a YA novel. It's a broad definition, but one that makes arguing over the parameters less prominent. (Yes, I know I was the one who brought up those parameters in the first place. :P)
After reading their blog post about how to qualify things, I disagree with their methods a bit and favor on the overwhelming response from the comments therein.