I loved the opening cameos. Only points not yet noted:
1. Was Burt Ward's Robin walking Ace? (Batman's dog; he shows up occasionally in the comics.) If so, that's a nice touch, unless you realize what happened to them. This is one of the great six-degrees-of-DCeparation episodes, as a crossover could be.
I'm also glad they got the fan service out of the way early, and with enough of a dark twist that it prepared for the more serious parts of the episode.
2. I wondered if that was Wil Wheaton (guy with the end of the world sign, if you missed it) reprising one of his other roles, or if it was a new character for the show. I'd love for Earth-38 also to be The Big Bang Theory universe, and it's that Wil Wheaton who got a call from one of his NASA friends that things were going to go badly, so he tried to warn everyone. That, or it's the Best. Tabletop. Crossover. EVER. Those aren't the only two options, of course.
3. I liked how Supergirl and Superman figured out that solar panels could be temporarily recharged with heat vision.
4. I liked how Lena tried to draw the line between antagonist and (super)villain--she sees herself as a good person.
I'm also wondering which Arrow dies, and both where and how they're going next, so on those fronts, well played, show.