- Complex characterizations and plots--although it works as such, it's arguable if the show's creators are really targeting kids for these shows so much as they're targeting adult and subadult fans of the comics and characters.
- Combination of some humor, ensemble cast, and a fairly dark premise. Maybe not Dark Knight dark, but certainly not the old Superfriends type bright n' polite either.
- Mostly (although there's an exception to this) packed with pretty solid action that avoids the rather obvious "let's not show the kiddies any violence" syndrome. This doesn't mean that they're bloody and gruesome, just that you can tell that these characters are actually fighting in their action scenes, and that they actually hit each other and stuff.
- They all ended on cliff-hangers, and then were cancelled before being able to continue.
An interesting comparison can be made, especially with the second seasons, of Young Justice and Avengers because both primarily deal with a covert invasion of Earth by aliens, and how the superheroes deal with that.
It also managed to put most of the actual Justice League members out of pocket for the entirety of the season, which made the fact that the teenaged (and early 20s) "Team" of Young Justice and Teen Titans style members having to solve all of the problems more believeable. It also managed to do away with the inconvenient (from a story-telling perspective, anyway) fact that Superman is just too good, and therefore automatically "kills" any story in which he appears as a major character (luckily, writers tend to mostly conveniently forget that he is at least as fast as Flash when he wants to be. They also tend to forget his ice breath, and just make him a really strong, flying, heat-vision blasting hero. To make this official would bring Superman down to Thor-like levels--still one of the "big guns" of the League by any stretch of the imagination, but not so completely over-the-top that he ruins everything that he's in.)
And finally, for someone like me who's not terribly invested in (or knowledgeable about) the DC Universe, it is an interesting arena for me to get to know more characters--both heroes and villains. Some of them are kinda silly (Toymaker?) but still--it's nice to see the others who aren't. It's also funny to see the obvious correspondances between Marvel and DC characters in many ways.