I think the "serious"vibe is just because they're all posed for the poster. M'gann is the only one who doesn't look pissed off. She's the Holger Fath of YJ.
I'd bet Artemis is basically going to be Ravager in all but name.
What makes you say that? Her costume pretty clearly evokes Green Arrow (and thus Speedy [though a more traditional look than the Red and Yellow outfit she's been wearing in the comics), and her earlier working name was a YJ-era accurate Arrowette, both of whom are more appropriate characters for a kids' TV show than Ravager.
Arrowette was pushed into superheroics by her mom, and eventually learned to become her own person. The newest Speedy is a timid, self-conscious rookie who must gradually learn to assert herself. Ravager is a victim of parental abuse who seeks bloody vengeance, and must learn to follow the group's rules. Two of them are good template characters for a childrens' TV series. One of them isn't.
(Yes, I know BtAS/JLU could've gotten away with Ravager, but it was older-audience fare that aired [in some markets at least] as a prime-time series. A show with teenaged leads is going to be a family-friendly show. Expect this to air on Saturday mornings.)
Ravager may well show up later into the show, but they wouldn't put an anger-driven avenger as a lead in the pilot.