Even as a kid, I didn't find the earlier incarnations of the Scooby-Doo franchise to be that great, I found them lacking in energy and at best semi-entertaining. But they are something that a lot of people, young and old, remember fondly and well, and there's experiences assumed to be universal, like loathing Scrappy-Doo (as a kid I actually appreciated his presence for shaking up the formula, but I can understand why he would be grating now).
I also speak from the perspective of a classic Hanna-Barbera fan; for a lot of people (probably less interested in animation than we), Scooby pretty much IS Hanna-Barbera, and when I think of H-B, I think of Huckleberry Hound and the other cartoons hewing to that visual style. For me, the appeal of H-B is tied very strongly to the studio's original house style, and the older Scooby shows don't really hew to that (geometric stylization wasn't really Iwao Takamoto's bag anyway, with regard to the character designs at least - and yes, I know, things like Jonny Quest and their superhero shows don't hew to it either). I've seen a bunch of different explanations for why Scooby still endures - Shaggy and Scooby have great chemistry (http://yowpyowp.blogspot.com/2014/06/farewell-casey.html), it's about kids looking for truth (http://comicsalliance.com/ask-chris-81-scooby-doo-and-secular-humanism/) - but it doesn't really hook me.
I think the show had the right amount of spooky atmosphere to entice younger audiences, with an equal balance of goofier slapstick to keep them entertained. In that regard, Scooby is like a much less sophisticated television equivalent to Disney's Haunted Mansion.
Although I'm honestly sick of the original Where Are You, and you can blame years of Cartoon Network's overplaying it. Even when I see some of the series now, while I do find elements that are unique to it, I feel unimpressed. And this is coming from someone who still genuinely enjoys The Flintstones and HB's earlier "funny cartoon" material.
Although WYA did have some great backgrounds to look at. They helped to shake things up a little from the static designs of the characters themselves.
It's more fun to analyze the show than to watch it.
https://www.youtube.com/watch?v=vhnjQfERXv0
Daphne's voice actor has passed away. (http://ew.com/tv/2017/12/19/heather-north-dies-scooby-doo-actress-dead)
Rest in peace, Heather North.
Daphne was never the most developed character, particularly in the original, but she did a good job of bringing her to life anyway. RIP
R.I.P. Joe Ruby (https://variety.com/2020/tv/news/joe-ruby-dies-dead-scooby-doo-1234750948)
R.I.P. Ken Spears (https://variety.com/2020/tv/news/ken-spears-dead-scooby-doo-co-creator-1234825813)