As with "Initiation" two seasons ago, "I Am Legion" hits the ground running but really doesn't give us the degree of characterization that episode did.
The similarity to "Initiation" is that the "Legion of Doom" (note that Grodd doesn't refer to them by that name, or any name) just kinda shows up without much explanation A big crowd of faces that you can barely make out, and a quick explanation for why they're all together. The villains must feel rather offended they're all there just to impress Lex Luthor. And who puts a big honking door like that as your entryway? Or maybe it's the "Dramatic Reveal Door" plot device?
Unlike "Initiation," the key players (pardon the pun) aren't really introduced. Unlike Green Arrow, Captain Atom, and Supergirl, new villains Dr. Polaris and the Key don't make much of an impression. A second-rate Magneto and a guy with a key-gun that unlocks guns and fires energy beams - whoo. Grodd is his usual slimy manipulating-others self and Lex seems to still be working on reestablishing the Luthor/Brainiac alliance. But we really don't get much of a feel for the Legion at this point.
The Leaguers are well-rendered although the establishment of the JL Embassies is rather glossed over. How many are there? Why is a Thanagarian, an American, a Brazilian, and a Icelander running the one in (presumably) Metropolis? Flash and Hawkgirl are nicely personalized and the episode focuses on their big sister/little brother relationship which has been hinted at in the past. Hawkgirl does seem a little more forebearing with the guy, after the events of "Divided We Fall" and saving him from the Speed Force. Fire is okay and it's at least interesting to see that some Leaguers look up to the Flash as one of the "senior members."
What really made the episode is the Blackhawk angle, and the fact that we get an unexpected continuity hook-up to "The Savage Time." There Chuck didn't even have dialogue, so it's kind of funny to see him become the main representative here. He gets some time to shine and holds his own against kitschy giant animal-robots and super-villains alike.
The only problem with the Blackhawk subplot was that it seemed to fit on a bit awkwardly to the general plot. The memorial at the end (and Hawkgirl's and Flash's reactions) was very nicely done, but it doesn't really fit with the rest of the episode.
Overall a nice kickoff to the season - it set up the Legion of Doom, but whether they can sustain a story-arc to rival Cadmus remains to be seen.