With 15 eps done and 5 to go, here is my standing of the major characters who are/were on the team based on season 2. Bare in mind that my opinion may change based on what happens in the next 5 episodes.
Best new character:
Blue Beetle: the season has basically been about him and the writers have done a good job of making him likeable. I also think it was a brave decision on their part to make a relatively obscure character be such a major part of the season.
Impule: I think the Flash family is naturally likeable. Also since he's written to be a loyal friend to Blue Beetle, he comes off even more likeable.
Arsenal: I just think Roy is very well voiced. I definitely prefer the older Roy but this version is pretty cool too.
Worst New Character
Lagoon Boy: Reasons are pretty obvious but it seems that he's just been written to be a hothead who gets humiliated. There's nothing much to his character outside of being Miss Martian's boyfriend.
Bumblebee: shown to be rather cold and distant towards her boyfriend to the point of being downright rude until 'War'.
Mal Duncan: a character that has served very little purpose. His storyline with Bumblebee has not got enough screen time to be invested in nor does he provide much functionality outside of that relationship.
Most Improved Character
Superboy: I was pretty annoyed with his constant angst and anger last season. But this season he has come off fairly likeable. He seems to have a balanced head on his shoulders and has clear distinction between right and wrong.
Aqualad: he was a bit boring in season 1. Too much of a straight arrow. But he became a badass in season 2 and his story has been the most interesting one to follow out of all the lead characters of the show.
Most Missed Character
Zatanna: I honestly really liked Zatanna last season. She added a spark to the second half of the first season. Also, she was a pretty sexy animated character. Which is why I was quite disappointed when she was shown as part of the JL in ep1 of this season. She hasn't been around much with her most substantial role being in the 'Despero' episode.
Red Arrow: He was the ultimate badass in season 1. Arguably my favorite character. His story sttarted off promisingly this season but was resolved too quickly. He has been sadly MIA ever since his "original" version has popped back up. Its a crying shame!
Kid Flash: I am a huge fan of Wally in the comics, in JL, JLU, and in YJ. Sadly he's not in the comics anymore which is why I have felt really pissed that Wally has not got even one proper full length role in an episode this season. His most substantial role was pretty much supporting in 'Bloodlines'. Whats even more irritating is that I've loved the more mature Wally that we've got only a glimpse off this season.
Characters that I haven't changed my opinion on
Miss Martian: She was incredibly annoying last season. Her actions this season make her quite unlikeable. Plus, I did not like the fact that she dissolved into self-pity when she realized the error in her actions. However, despite her unlikeability, she remains probably the most well developed character on the show.
Artemis: she was pretty cool in season 1 and she has been pretty cool in the two episodes that she's gotten so far.
Most Disappointing character
Nightwing: the only part of the five year jump I was excited about was seeing Nightwing since he was very much a recurring character in the Bruce Timm universe and I wanted to see him more in action. However he has been majorly disappointing. Not only have his decisions made little practical sense but he has put younger team members in harms way. What's more, he has done so mostly at harms length while sitting in relative comfort. He has been out on the field twice this season and one of those times was planned. We haven't got to see his great detective skills or his martial arts skills. I hope he turns it around before the end of the season.
Characters I don't really care much about
Pretty much the rest of the team. Most of them are underutilized or just aren't interesting.