I think I've figured out the problem with Supergirl, and it's a problem that various versions of the character have been victims of. The big problem is in her name, "Super-Girl." People like Jeph Loeb write Supergirl as if her name were the sum total of her character. She's super, like Superman, and she's a girl.
In other words, they see an S-shield plastered on a pair of breasts and nothing else.
So we have some versions of Supergirl who are defined by whom they're dating. We have Supergirls who spend their time designing new costumes or crushing on Nightwing. Many writers seem unable to give Supergirl a distinct personality that can't be described as "well, she's super, and she's a girl." It's not too difficult to see how writers fall into that trap. If you make her too strong, willful, opinionated, and self-sufficient, she becomes Wonder Woman. If you make her too naïve and wide-eyed, she becomes super-damsel-in-distress. If you make her too grown-up and independent, she becomes Superman with breasts. If you make her too young and lacking guidance or motivation, then she becomes a sidekick. Writing Supergirl is walking a tightrope line over a deep pit of utter redundancy. If you make her too extreme one way or another, she becomes some other character, and there's nothing to make people buy her book over someone else's, except for the S-shield.
This problem is doubly compounded in the modern era, because of two other characters who have filled aspects of Supergirl's life: Power Girl and Superboy. Superboy became a viable character through years of well-crafted stories by Kesel and Grummett and Co., based around a central idea of "NOT the adventures of Superman when he was a boy," tales about a character who had many of Superman's powers and was learning to cope with the mantle that he'd taken somewhat selfishly, but also used his powers to have fun, pick up girls, and be more than a little self-centered. It was a Superman growing up around bad influences, without the careful guidance of people like the Kents. Kon-El has grown up a great deal since his first appearance, and that's a testament to the character's staying power and depth, but any Supergirl who displays the same teenage tendencies toward self-centeredness, or take on a more brooding attitude, is going to read like something we've already seen before.
Much of the same can be said for Power Girl. She's another character who has achieved a startling amount of depth, but more through a committee of people who loved her character and tried their best to give her a personality and a place to belong. Power Girl has had to deal with various crises of identity and how to cope with her incredible powers (fickle though they may be). Today, she's confident and sensual, but still a little insecure.
With those two, plus Wonder Woman and Superman himself, it becomes that much harder to write an interesting, original Supergirl. This isn't to say that it's impossible, mind you, but it seems like the best portrayals of Supergirl have needed some kind of hook, something that sets them apart from the rest of the Super-family.
The Secret Weapon: When Supergirl first debuted, she became Superman's "secret weapon," ostensibly so he could bring her out only when absolutely necessary, allowing time to train her in secret, and for her to grow up semi-normally. I admit I haven't read much from this Supergirl era, so I don't know how good the stories were, but seeing how the modern Superboy chafes at spending any of his time in a civilian identity, I can imagine some good drama developing from a situation where even Supergirl's public (read: superheroic) identity is a secret one. Rebellious teenager Supergirl uses her abilities in secret to help others, in-between fighting world-shaking threats at her father figure cousin's beck and call. Like a more cosmic Smallville, except with Superman as semi-bad-guy, the parent/authority.
The current Supergirl looked at first like she'd go this direction, but then Superman introduced her to the superhero community at large on the next page or so.
Mae: Not much of a Supergirl, after she arrived in our universe, the Matrix Supergirl went through a period of readjustment to humanity following a breakdown. The Kent family raised her secretly and helped her become a human being again, and what's more, a superhero. In the meantime, though, she was childlike, insecure, and repeatedly looking for acceptance and identity, whether or not that identity was her own. A thankfully short-lived time for Supergirl, but it helped establish that version of the character, and it gave her quite a bit of personality and motivation.
Earth Angel: I would say that Peter David's take on Supergirl was probably the most original and interesting the character has ever been. By distancing her both from her confusing past as Matrix as well as from Superman himself, David was able to give her her own mythology and supporting cast. He linked her in with Hebraic mythology, Shakespeare-era fairy tales, and a whole bunch of DC's less well-known magical elements, giving her a really rich tapestry to play from. Divorcing Supergirl from Superman in terms of Kryptonian heritage, powers, science fiction, and even wholesome past and attitude, turned out to be the best decision ever made for the character.
Of course, not all the hooks have worked. For instance...
Supergirl from Another World: So, the Time Trapper created a pocket universe for the Legion of Super-Heroes to travel to, in which they encountered a Superboy who would go on to become Superman, who served as the Legion's inspiration. That Superboy accidentally released three criminals from the Phantom Zone who laid waste to his Earth. The Lex Luthor of that Earth, a hero, cloned Lana Lang and gave that clone super-powers, including telekinesis and shape-shifting, and sent her to the real universe to recruit Superman's help. After that mission, she came to the real universe to stay, and became Supergirl (eventually). And that's the simplified version, that's not even including the gray Superman and Draaga.
Lexy's Girl: Same Supergirl, slightly later. Lex Luthor faked his death and returned in a cloned body, claiming to be his own illegitimate Australian son. This new Luthor appeared to be a decent guy, and eventually he ended up dating Supergirl. This led to her being defined entirely by the fact that she was dating Lex, to the point of defending him to anyone who would speak poorly about him, and then to storm away in a huff. When she found out that he'd been both playing her and cloning her, she got understandably upset, and the writers decided to give her a temper and a mean streak. Now, it was an understandable situation for going a little nuts, but it didn't make her any more interesting.
Cir-El: Even if her name didn't sound like "cereal," Cir-El was easily the dumbest, worst Supergirl ever. Worse than the one Jimmy Olsen wished for. The whole "I'm your daughter from the future" thing was lame, and we all knew it would end up being totally false. Couple that with one of the most obtusely confusing stories in recent memory, and a costume so ridiculous it made the Red Bee look like a fashion plate, and you've got the makings of the world's worst Supergirl.
Unfortunately, the current Kara's gunning for that very same title. Inexplicable Red Sun bursts vs. Greater power than Superman? Giant amorphous "S" vs. Midriff top? I'd still give it to Cir-El, but just barely.
So far, the newest Supergirl has been a central figure in something like 12-15 issues, and she still has no personality. She's been turned evil, sure. Twice. The extent we've seen of her personality is that she wants acceptance, doesn't like Batman, and has a crush on Nightwing. She's also got a fairly hot temper. Seems like she'll fight just about anyone, anywhere, at any time, if they rub her the wrong way.
Oh, and claustrophobia. Can't forget that. She's not brash and impulsive like Superboy, she's just easily provoked. She has an uncanny ability to make everyone around her instantly hostile and aggressive, even when it's wildly out of character. That's not good characterization. Teenage characters often get enveloped in weird drama, but usually because of doing typical teenage things or having typical teenage personality quirks. Supergirl's not a typical teenager in any fashion (but we'll get to that).
So Loeb has given us a character who is interesting mainly because of the mystery surrounding her--is she good or evil? Is she really Kryptonian? Why is she stronger than Superman? Why does Krypto hate her? She's falling into the "Wolverine Syndrome" pit already, avoiding the various pits of redundancy in order to just succumb to awful writing. She has no personality to speak of, so once these mysteries are resolved, there's nothing left to make her interesting. That's not giving us a powerful Supergirl character.
Of course, the art hasn't helped. Drawing this ostensibly 15-year-old girl like she's a 23-year-old anorexic supermodel (who can somehow find spandex jeans) has sexualized and matured her, and either Loeb and other writers are picking up on that, or they're just writing her poorly, because since she first crawled out of her spaceship, she hasn't once spoken or acted like a teenager. Maybe it's because she's an alien and has been indoctrinated by the Amazons, but giving her two different alien heritages has only served to utterly divorce her from humanity. You may recall me saying above that Peter David was the best thing to happen to Supergirl ever, and that's in no small part due to the fact that he helped forge a stronger connection between her and humanity. Making her alien to humans, alien to modern civilization, and alien to the superheroing world, without developing any supporting cast to connect her to anything at all, has made her generally inaccessible. I trust Rucka, who seems to understand strong women and teenagers both, to make the new Supergirl readable, but in another writer's hands, I could easily see her becoming a radioactive living deus ex machina. No writer would know what to do with her or how to handle her, and she'd only appear to help people out of problems or further complicate them.
And it's just that sort of situation that led Peter David to take her away from the Superman cast and give her an identity of her own. I love the way comics keep regressing under folks like Loeb and Waid; characters who were interesting and deep are replaced by characters who are more like they were in the Silver Age. Yay.
The best thing that could be done for Supergirl at this point would be to give her a unique supporting cast. Take the Justice League-types out of her life for awhile, and let her get some guidance from others. Maybe she should hang out with Natasha Irons and Steel. Maybe she should bond a little with Superboy, once he stops PMS-ing. Maybe she should find herself a couple of other teenage girl superheroes who can give her a secret identity and help her deal with the double-life. Might I suggest Cissie King, Greta, and Anita Fite, late of Peter David's (much missed) "Young Justice" series? They also have ties with Wonder Girl, and it seems like Supergirl ought to find a good friend in another superpowered young lady with Amazon connections, despite how uncharacteristically jealous Cassie got in that recent Supergirl issue.
I expect Rucka to do good things with this version of Supergirl. I just wish Loeb had given him more to work with. Like a supporting cast, or an established personality. Instead, we have superpowers and girlie traits, and that's well below the bare minimum of what should be expected from a year's worth of new Supergirl.