That's a really good questions about what a good bark is. I guess you knwo when you eat it! It doesn't matter if it's crunchy or simply firm if it doesn't taste good. Looks would be important too I guess.
Dang, this may be harder to do than I thought.
I suppose what I was thinking, was a good tasting, firm bark that stood out. I ahve had some "crystalline" type bark like barbefunkoramaque mentioned. It's not crunchy, but firm, flavorful with extra-flavorful crystalline "bits". I really enjoyed that bark, but I suppose it does not have to be crystalline to be good.
Damn, now it's turning too subjective.