On his blog, Mark Evanier -- who worked on various seasons of SCOOBY-DOO, and who had a hand in the development of Scooby's nephew Scrappy -- muses on the appeal of SCOOBY-DOO:
The appeal of the show was always, I thought, in the easy-to-watch energy of the characters, especially Scooby and Shaggy, and also in a certain cumulative effect. I don't think you could ever be much of a fan of Scooby Doo if you watch the occasional episode. No one episode is particularly memorable and some of them are quite silly and contrived, even by Scooby Doo standards. But watch enough of them and...I dunno. Maybe it's your senses atrophying. Maybe the characters become so much a part of your family that you'll forgive them anything.
And he also ponders why fans thought much less of Scrappy:
Scrappy did exactly what he was supposed to do: He got Scooby Doo renewed for another season. I don't think he was a good addition to the format and the fact that he could talk, while his Uncle Scooby sorta couldn't, tore the already-frail "reality," to use that word in the loosest-possible manner. Then again, the underlying premise of "there's no such thing as ghosts" was shredded somewhat during the seasons that the show had guest stars and so Scooby was teaming up with Speed Buggy (a talking car) and Jeannie (a genie). Later, of course, they gave up altogether on the notion that the supernatural did not exist and had Scooby and Shaggy chased by real werewolves and mummies and space aliens.
Actually, as a kid, I don't think I ever really got the premise in SCOOBY-DOO that the supernatural didn't exist. I think I did wonder why in each episode the "ghost" would be unmasked as some non-supernatural real estate developer or smuggler, after the bulk of the episode showed that purported fake performing feats possible only of some supernatural creature -- and often staying in character despite the absence of any logical reason to do so. Besides, it was a cartoon -- and in a cartoon, everything was magical. Drawings were moving! (Just barely. But still.)
But yes, I was one of the kids who despised Scrappy, and found it impossible to watch an episode involving him. I took some pleasure in the live-action SCOOBY-DOO movie of a few years ago, which featured the indelible image of Scrappy piddling on Daphne.