I will not pretend that Scooby Doo, Where are You! is high art, or the greatest animated series ever; however, there is a reason this is a classic and so fondly remembered. This hits so many strong points, from the eerie music, the oppressive yet whimsical tone, and the creepy and sometimes scary villains. Perhaps the most daring has to be the choice of such a dark color scheme(especially for 1969.) All this contributes to the wonderful mood that permeates Scooby Doo. It is a shame that many future Scooby Doo series could not equal the magic that was there in 1969, because they got so much right. The first season is by far the best, but do not knock the second, both are great.
The Gang(Shaggy, Velma, Daphne, Fred, and of course Scooby Doo) travel around in their van(The Mystery Machine) and investigate seemingly supernatural events. This allows each episode to have it's own spooky locale with a wacky spin on the horror genre. Primarily Shaggy and Scooby Doo are the main focus of the show, acting as comic relief to the goings on. Lastly by encouraging critical thinking, having rational explanations, and showing them confronting their fears teaches good lessons to children, who are the target audience. Not that adults can't absolutely love this show, as the superb mix of comedy, mystery, horror, and atmosphere all make this a great cartoon for all ages!
If you liked this show and wanted more Scooby shows, I recommend, The New Scooby-Doo Movies, The 13 Ghosts of Scooby-Doo, What's New, Scooby-Doo?, and Scooby-Doo! Mystery Incorporated. They are all a little different, but are about as good if not better.