Cartoon Network - Image #415,061

The point of the picture isn't that there aren't good cartoons anymore. Is that there's a LACK of good cartoons nowadays.

Regular Show, although I don't watch it much, has really taken some risks with their show (that paid off), and I never watched Gumball. But Adventure Time, in my opinion, got better as the show progressed. The earlier episodes weren't very good, but the new ones have really interesting art, characters, good animation and the jokes have gotten kind of surreal, which is more suited to the show. And it has kind of a continuity to it now. There are a lot of allusions to earlier episodes now and ongoing plot lines.
So I wouldn't say they're all bad; people just aren't willing to accept new things sometimes.