This episode was absolutely awesome, easily one of the best so far for me, up there with Dawn of the Deadman, Terror on Dinosaur Island and Fall of the Blue Beetle. Despero was a truly great villain here, and I loved how the themes of the teaser (no matter what powers [or lack of] you have it only matters who you are) carried on to the main ep.
Doctor Fate's voice was awesome, I love the way the Library of Infinity is portrayed, it was also good that it explained why Batman was there, giving Fate lessons on hand-to-hand combat.
The main event was even better, the fight with the Cavalier is one of my all-time favourite B:TB&TB scenes so far: "Some where Shakespeare is spinning in his grave"
G'Nort was greatly portrayed I felt, I always loved his character as he was the one who was never smart nor ultimately that great at fighting, but he always had the spirit and goodwill. Plus his lines were flipping hilarious:
"On Labor Day, On Halloween Night, No nose hair shall escape my sight!"
"Hang on! I forgot about my cheatsheet! *reads Green Lantern oath off his wrist*
Guy Gardner is great as always, as previously mentioned the JSI homage was excellent. I also liked the way Sinestro was portrayed, he sort of mirrored Despero but felt he was working for good, trying to rd his sector of evil. It establishes him well as villain who believes he is doing the greater good.
So yeah, fantastic episode in my eyes, really fun. I hope we get more ones like this in the future, with expanding themes and teamups with more than one hero.