I had figured that my expectations were low enough that I wouldn't be disappointed. But it turned out that there was one way to disappoint me that I hadn't thought of:
I'm not quite sure how to phrase this, but I was disappointed because the movie was laughing at the Scooby-Doo tropes, instead of having fun in the Scooby-Doo tropes. Some examples of what I mean: the episode of Johnny Bravo that parodied Scooby-Doo was an example of laughing at the pattern; A Pup Named Scooby-Doo, on the other hand, was an example of playing within the pattern (and turning all the knobs up to 11 for maximum silliness).
This distinction explains a lot of the things I found disappointing with the movie. For example, I didn't like the portrayal of Freddie as a smug, self-centered git; I didn't feel that was true to the source text. Even though I laughed at the fart jokes, I didn't feel that they were faithful to my understanding of the characters.
The climactic revelation of
It did do a good job of being a parody, though. Sarah Michelle-Gellar's horndog leer was particularly fabulous. And I liked the references to Buffy the Vampire Slayer, Night of the Living Dead, and other references.
In conclusion: it was fine. I don't feel a need to see it again.