Contested Sequel: Reinventing Comics was highly controversial when it was released in 2000, with many arguing that Scott was overreaching in his predictions on comics' future. The ideas he proposed for the Infinite Canvas were and are especially contentious, as even to this day it's written off by many cartoonists - with someexceptions - as downright impractical. Making Comics, on the other hand, was much more warmly received in comparison.
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.