Batman states that the Black Mercy has eaten through Superman's costume. When the plant is removed Superman's costume is intact.
If Mongul was planning to take over Earth, then why was he still in the Fortress of Solitude? Shouldn't he have just taken care of Superman once and for all and moved on?
And if we assume that Superman's birthday is so well-known that aliens send him gifts...why don't any of the other Justice Leaguers give him anything? Some of us might want to see what the Flash would have given him. Or are we supposed to believe that a bunch of aliens we've never seen Superman help know his birthday date, but his teammates of 2+ years don't?
Does Superman really publicize the date of his birthday? In the pre-Crisis comics he saved alien worlds every other day (so did Batman in the 60's...), but post-Crisis and in the various animated series he hasn't been quite so active or forthcoming. Maybe his friends Batman and Wonder Woman know it, but do alien planets really know it widely enough to teleport gifts to him? But if alien planets don't know when Superman's birthday is, why is he so accepting of the supposed birthday present?
When Wonder Woman calls out to Batman after her fight with Mongul, she's missing the pupils in her eyes.
How do aliens know where to send Superman his birthday gifts? His Fortress of Solitude is...well, a place of solitude. Secret. No one on Earth but his closest friends knows about it. Or does Superman use a teleportation forwarding service?
Batman: A little sharp on the turn, don't you think?
Wonder Woman: Sorry if I scared you. (Batman just stares)
Batman: What'd you get him?
Wonder Woman: I'm not saying anything – he'll hear and spoil the surprise.
Batman: (whispering) He can hear that too.
Batman: He's not the easiest person in the world to buy birthday presents for. (holds up envelope)
Wonder Woman: Bruce... you didn't get him a gift certificate?
Batman: No! Cash.
Mongu: You don't understand. He was the only obstacle in my way. The rest of you are already dead...
Mongul: How nice of you to volunteer... to be the first of your race to die!
Mongul: Oh dear, is that a neuro-impactor? I didn't know they were still making those.
Mongul: I'd advise you to try the plasma disruptor. It's more of a woman's weapon.
Superman: Do you have any idea what you did to me?!?
Mongul: I fashioned a prison that you couldn't leave without sacrificing your heart's desire. It must have been like tearing off your own arm.
Mongul: Happy birthday, Kryptonian – I give you oblivion.
Mongul: You know for a moment there I almost believed you were going to kill me. How stupid of you to hesitate like that. Not a mistake I'll make, I can assure you.
Christopher McDonald also played a future, aged version of Superman in Batman Beyond, in the episode "The Call".
Eric Roberts reprises his role of Mongul from "War World." Christopher McDonald reprises his role of Jor-El from Superman: The Animated Series. Mike Farrell (briefly) reprises his role of Pa Kent from Superman: The Animated Series and the JL episode "Comfort and Joy."
Krypto makes a brief appearance in Superman's dream and General Zod gets a name check (see Recap).
Kal-El's wife's name is Loana...presumably a combination of the first names of Superman's two most notable love interests, Lois Lane and Lana Lang. Also, Loana is voiced by Dana Delany, who has voiced Lois Lane on previous episodes of Justice League and Superman: The Animated Series.
Wonder Woman's famous Invisible Plane appears (sort of) for the first time in the series.
Mike Farrell is credited as Pa Kent but isn't seen on screen. During the fantasy sequence, Jor-el says a line in Pa Kent's voice right before Kal-el and his son go upstairs. Christopher McDonald says, "Go on up with your father, Van," but Mike Farrell finishes, "I'll be along in a few minutes." If you turn the closed captioning on it even says "[Pa Kent's voice]" when he says the line.
Braniac: Floating Robot
The flying Brainiac looks suspiciously like Herbie, the robotic major-domo that appeared with the Fantastic Four in their animated series and later was introduced into the comic book.
Young Bruce: "The Mark of Zorro"
On the play sign outside the theate that Bruce and his parents came out of in Batman's dream sequence, you can see a sign that says Mark of Zor.... The 'O' is missing, but you can clearly tell it was a take on the character Don Diego de la Vega, or Zorro. Presumably the reference here is to the 1940 version starring Tyrone Power and Basil Rathbone, unless the Waynes were real big on the silent 1920 version with Douglas Fairbanks.