Adventure Time uses its art more skillfully than most other minimally-designed shows.
A show like The Amazing World of Gumball is horrendous. I'm not against Flash if there's some thought put into it, but this show makes so little effort with its art that it uses live-action backgrounds instead of anything resembling a coherent style.
Cartoons never were just for children. And neither is it a genre. It is an artistic medium where the use of caricatures are used to entertain or educate, with animation bringing said caricatures to life.
With such contemporary cartoons as The Simpsons, Family Guy, South Park, and the Adult Swim line-up, it's hard to dismiss the animated cartoon as only kids' fare. Cartoons can be for anyone. It's pure common sense.