Crowning Moment of Awesome: Understanding Comics can be seen as one long example of this for comics as a medium. McCloud expands the timeline of comics/sequential art nearly to the dawn of history, shows just how versatile a medium it really is, and demonstrates not only that it is a valid art form with its own unique language and grammar, but also, through the book's very existence, demonstrates sequential art's power for serious non-fiction as well as storytelling and humor.
Sequelitis: McCloud seems to have concentrated his best ideas (such as the "big triangle") in the first book, with Reinventing and Making being less ambitious by comparison.
Uncanny Valley: The big triangle shows the varying stages of simplifying and abstracting a human face. YMMV on where the valley actually lies on it.