For one, the number of details is incredible and the attention to them, phenomenal. Example: watch how they aged the parents, physiologically, both of them. That will show you what I mean.
The graphics are flawless, except for wet hair. Pixar hadn't yet found a solution to that problem. The dialogs and the scenes will capture the attention of everyone since you can have fun with the slap stick stuff and the action, or see the problems the couple has as well as each individuals. Many parents will recognize recurring household frictions and dynamics.
The scenario is rich in adventure, super heroes, and some villains. The architecture and designs will make you think of "modern" world imagined during the 60s. It's quite impressive. The drawing themselves have a particular style that is very attractive and different from what's out there.
By the end, you'd wish to see ten more movies in that same world, that's how interesting it is.
It's a must.