Young Justice is one of the best looking western animated series I've seen in a while, with great animation and character designs. But... it also has problems. Lots of problems.
My biggest problem with YJ is the characters. Miss Martian ("Hello Megan!") and Artemis are the worst offenders as far as bad characters go. Miss Martian provides the girly pink ranger character, who'se insecure and desperate for everyone to get along. This manifests as being dependant on male characters, baking cookies, and trying to get Superboy in bed. Then there's Artemis, who tries to balance things out by being Tough and Badass! Unfortunately she's also rude, incompetent, and awkwardly shoehorned into the team. Batman forces her into the team, so Artemis is here whether the rest of the characters like her or not.
This comes to the second problem for characters. The other characters have their moments- even though Aqualad usually has the emotional range of a brick wall, he gets fleshed out a bit in Downtime. The problem is their interactions with one another, which seem unnatural sometimes. When Artemis joins, she is very terse and pretty rude. She then screws up and lets the villain escape. The team's response? Universal acceptance! The only exception is Kid Flash, who finds himself caught up in the most obvious love-hate relationship ever. If it weren't heavy handed enough to begin with, shortly after Artemis joins the team, Dr. Fate makes incredibly unsubtle comments about how he thinks Flash should bone her. It's painful to watch.
Stilted character interactions like this hurt everyone. While some of the characters come across well at points, as soon as they start following the great invisible script in the sky, they stop being characters and start being robots. Another example is one where they decide to make Aqualad the temporary leader, until Robin can man up and be the actual leader. Why? The scene in question fails to make a convincing argument as to why either of them should lead, Robin being portrayed as an immature kid, Aqualad as a brick wall. The reason is, of course, the invisible script in the sky.
Hopefully the series will get over this and actually start to develop some of these characters into interesting people, but it's got a long way to go. Still, if they at least drop "Hello Megan" they'll be off to a good start.