The villains are frightening. The characters are fleshed-out. the plot is dynamic. The art style is unique, and its universe has depth. Put simply, this is Scooby-Doo living up to its fullest potential.
I know my Scooby. I grew up with all the incarnations of the show, and while it's generally held a place in my mind as being fun, I probably would never have considered myself a true fan. I didn't doodle Scooby on my class notes.
But some genius came along and changed the rules. This show is true expansion, not a retelling of the original story. Not a remake. It's not trying to be hip, or modern. It's trying to be iconic; it's trying to be bold. They flashback to scenes straight out of the original series, and build their universe brick by brick, staying true to the core elements of Scooby (including the ridiculous slapstick), while introducing intrigue, fear, tension, development - and humor, like you've never seen in Scooby before.
I think the moment I realized that I might be watching something incredible came along when Velma was giving a tour of her family's Spook Museum, and casually resting in the background was the opening monster from the Scooby-Doo live action film. It never came up; it was just *there*.
That was a tell-tale sign of a groups of creators that loved their material - both good and bad. It takes guts to recognize the embarrassments of a franchise, and then continue onward, using elements of that embarrassment to produce something wonderful. Passion is oozing from the seams in this show.