There are some spoilers ahead, but I'll try to keep them to the first part of the show.
One of my cohorts recently
threw down the gauntlet (or possibly spandex glove) by comparing
Young Justice to
Avengers: Earth's Mightiest Heroes. As he pointed out, they are for different age groups, so it might not be entirely fair to compare them. While
YJ gets to have complex storylines full of brooding and woe (and veiled references to drug use, with Red Arrow's latest story arc),
A:EMH is much more about fighting bad guys and stopping cartoonishly evil schemes that usually consist of "taking over ____," revenge, and theft. This isn't to say that it's completely silly: Captain America still mourns over the loss of his entire world, Hawkeye and Black Widow have tension while going back and forth on just how evil Widow is, and at one point the Hulk is rejected and goes off to be alone. These moments are great, but they're fairly rare, because the show is about something else: it's about saving the world, and being mighty while doing it.