On the topic of slinky dog, I think the draw is that it's an outdoor coaster at a resort not known for outdoor coasters. There are five, maybe five and a half, depending on how you count Tron: Big Thunder, Barnstormer, Seven Dwarfs, Everest, and Slink. Of those, Big Thunder is awesome and iconic, but Barnstormer is a true kiddy ride, and Seven Dwarfs is too short, while Everest doesn't appeal to families.
Slinky Dog is the first outdoor coaster at WDW to give high-up arial views while being accessible to families with children. It also has a fun launch sequence that isn't too thrilling but has fun flashing lights that make kids smile. But more than that, it gets you up close and personal with a beloved character from a beloved movie, and he talks! It's a ride vehicle that talks!
It's popular because it offers fun views from the sky while being accessible to families, and it's also a character experience!
While I agree it may be too lightly themed, the "Andy built a roller coaster kit in his backyard and attached Slink to the track" idea doesn't really offend me, and seems like a decent justification for building an outdoor roller coaster in an immersively-themed area. What are the other options -- a runaway train? They already did that. Twice.
Now Tron's outdoor section, on the other hand...