Gotta disagree with the "snowflaking" mentionned earlier. I feel cartoons from 20 years ago were really more in the middle of the road, whereas now, there are the ultra safe ones and ones that try to go too far.
Old cartoons are a lot of Master of the Universe, Thundercats and... a lot of anime really. And it was mostly anime that went into truly edgy themes. Samurai Jack and Spongebob were more of an exception than the norm I feel.
Nowadays, as mentioned, cartoons that start of going for teen audiences (Adventure Time, Steven, Gravity Falls) end up clearly in more adult questionment territory. Not even talking about the rise of the adult focus cartoon after a few successes from Adult Swim. Even the last season of Samurai Jack went darker than before, whereas it clearly fell in the "gritty but family-friendly adventure" genre before, albeit very stylized.
Sometimes, I simply long for a good old "teen doing adventure" show. Di-Gata Defenders was a good one, that avoided repetivity and even made their "powers" useful for civilian life, kinda what you found in Avatar. But that's starting to date a bit. Maybe I'm not watching TV enough to find such recent middle of the road tho...^^