High Concept is definitely a "I know it when I see it" kind of thing. It' s a hard thing to define specifically. That said, my very clunky way of describing it is: What is the HOOK of the book? Does a short description promise potential images, characters, stories, etc...?
Think of it more from the reader's (or rather the buyer's perspective) Can someone "see" your book from a title? In just a few words? In a sentence? By "see" I mean do they get the idea quickly and completely. If you described it to someone would they be nodding their head while visualizing potential images or storylines? Then you're on the right track. If it requires more explanation and it's hard to know what the book is about without reading the manuscript and getting to know the characters then it's probably not as high concept.
Here are some examples. Just by title: THE MONSTORE by Tara Lazar and THE THREE NINJA PIGS by Corey Rosen Schwartz. You can make a pretty good guess what the book is about just by the title. It doesn't give away all the delightful surprises or the story arc, but you kind of get the tone and concept. BATTLE BUNNY by Mac Barnett and Jon Scieska is an example of high concept that can be summed up in a few words. A boy "rewrites" an existing sickly sweet book to make it more to his liking. Again, that description doesn't give away all the surprises and details but you "get" the idea behind it.
The Hunger Games is a non-PB example. Teens are forced to fight to the death in a televised competition. You don't really need to know about Katniss or the Dystopian history or the specific plot to "get" the concept. All those details come from the potential promised by the concept.
Anyway, that's my very inelegant two cents about it. Anyone else have more examples?