This was a terrific conclusion. Each of the other parts was more of a piece with its own series than with the others, but this featured characters and threads from
Flash and
Arrow as heavily as the LoT characters. Indeed, it felt more like a
Flash episode than an LoT episode, because Barry and Flashpoint were the catalyst of the whole thing, and because it featured Cisco so heavily and actually resolved his conflict with Barry in a different show. That's pretty remarkable.
Supergirl was still a bit underused, though -- until the end. Kara was more awesome in the last five minutes in civilian garb than she was in three episodes in the cape. But the Earth-1 heroes were as integrated as a single cast as they've ever been, and it was awesome. Felicity and Cisco are a great pairing, Oliver and Barry are pretty good too, it's good to see Oliver and Sara bonding again, Caitlin was a good adviser for Martin, and so on.
There were some terrific in-jokes. Ray saying Kara looked like his cousin is still cracking me up when I think about it. (A lot of people expected her to say he looked like her cousin. Neat that they went against expectations.) Felicity worked in a "This looks like a job for Supergirl" just moments after I said it to the screen (though I was talking about the Meta-Bomb). And when Felicity was suffering time-travel aphasia, the last thing she said was "Darmok and Jalad at Tenagra!"

Then there's Kara calling them "Earth's Mightiest Heroes" (Marvel's lawyers would like a word with you, Ms. Danvers), and who knows what else I'm forgetting.
I'm glad the solution they found to the invasion was nonlethal. It bothered me a few weeks ago that
Supergirl made an exception to Kara's policy against killing, and I was troubled by the preview descriptions saying that the heroes were looking for a way to "kill" the Dominators. So I'm glad the ending didn't go that route. Not only for Supergirl's sake, but because, really, the Dominators kinda have a point. They weren't here to destroy arbitrarily, they were here to protect themselves against a perceived threat. And technically Barry was the one who inadvertently broke the treaty. So killing them all for what they perceived as an act of self-defense (albeit an overly extreme and murderous one) would've been kind of a jerk move.
They've dealt with the alternate-Earths issue pretty much the way I expected -- not only giving Team Flash a way to cross to Kara's universe, but giving her a way to cross over and communicate with the other heroes on her own initiative. Which opens the door for more frequent interactions and guest appearances. I'm hoping that means someday we'll get to see Winn interact with Cisco, Felicity, or both.