"Starcrossed" begins as a spot-on take of the second part of "Secret Origins," bringing the whole thing around. Invaders show up, the League fights back, and...the League loses. Fortunately the Thanagarians show up to save them from the evil Gordanians. From there, the story takes a twist as the League helps the Thanagarians.
J'onn gets a nice moment one-upping the Thanagarian officer's scientific knowledge, but the thrust of the first part is Hawkgirl and Green Lantern. Two years in we finally find out the "origin" of Hawkgirl (meaning her story to J'onn in "Twilight" was a lie - he doesn't seem too worried about her dong so here), which shows a remarkable amount of restraint and a nice talent for long-term story-planning by the writers/producers. Unfortunately I can't really think of any foreshadowing of Hawkgirl's "real" role in the last two years. This'll be a little more important in part 2.
John gets to play the stoic ex-boyfriend and betrayed lover and we get the beginning of the romantic triangle that'll pay off in Part 3.
Things go amiss about when they should - any longer and it would have been kind of annoying. The League figures things out - Batman is of course naturally suspicious (and his investigation gives us a nice homage to Alan Napier), but the rest of the League aren't shown to be far behind. And the Thanagarians take down the League. It would have been nice if the fact they were being stricken down by their weaknesses was played up a little more, and where they came from.
Still, the Thanagarians having enough power to take down Superman establishes them as a proper threat. as noted elsewhere, the idea of casting all-Hispanic actors for the Thanagarians gives them a mildly "alien" touch while letting them stand out as a race.
Not much else to say - as a set-up, Part 1 works fine. There's more yet to come but Part 1 promises quite a bit and does so successfully.