I'm at somewhat of a disadvantage here in that I was around when this story was first released. As an adaptation it borrows pretty heavily, right down to the dialogue in places (i.e., "Burn!").
Mongul is back in a season that looks to give us a horde of returning villains in exchange for all the new heroic faces. It's a little odd when the writers have to latch on to old villains (Mongul, Amazo, Mordred, Morgana) so the audience won't be too badly thrown by new heroes. He's...okay here. Eric Roberts suitably voices him as evil incarnate, although I don't recall him being quite so powerful in War World. Superman beat Draga, and Draga beat Mongul - yet here Mongul walks all over Wonder Woman and still has enough left to go toe-to-toe with Supes.
The birthday idea makes a little more sense in the comics during the pre-Crisis era when this story was published, where Superman did have truly galactic fame. Here it seems a little odd (if not a full-fledged Goof) for him to be receiving birthday presents from alien admirers. It's not like Superman has a big rep outside of the Solar System, as War World itself demonstrates but is also consistent with the current varying series (which have rebooted Superman as starting his career recently in 2003, according to the Birthright mini-series).
So the adaptation here fails to answer that point. Instead it shortens the original story, omitting all the aspects where writer Alan Moore had the imaginary Krypton literally falling apart with race riots, corrupt right-wing politicians (including Jor-El himself!), etc., as Superman subconsciously destroyed the place in his efforts to break free. Instead we get a very basic, "I have a family - I lost them." boo-hoo story which in this case is hurt a bit by the half-hour nature of the show. The rushed aspect of the format means that we find out what's going on about a minute after the initial opening and credit roll.
Perhaps the story would have worked better to start with Superman waking up to his fantasy life, then show us Batman and Wonder Woman after the first commercial break. Showing us the sequence as it does reduces the impact of Superman's fantasy because we know from the beginning, before we ever see it, that Superman's in a hallucination.
So I'm on the fence on the reduced format - this story seemed too short here but I'm not sure it's really worthy of a 2-part treatment, either. This one just seemed...padded, with lots of fight scenes (and Wonder Woman being brutalized and outclassed) and not really enough of the Superman/fantasy sequence for the audience to sympathize with Superman's loss.
Batman is spot-on (except for throwing himself at Mongul early on - that seems oddly impulsive), although "cash" seems like a dumb present. Like Superman needs money, or can't crush a few coal chunks in to diamond if he did? Kevin Conroy never disappoints, and he packs a lot of emotion into his own fantasy and recovery. Wonder Woman really doesn't get a chance to do anything but play punching bag.
That leaves Superman. It's hard to be too impressed with his rage here - we've seen it before directed against Darkseid in a similar situation of metaphorical "rape," and the fact that Darkseid and Mongul could be cousins doesn't help much. George Newbern does a good job of packing some punch into his farewell to Timmy...umm, I mean Van El. I'm not sure if it's supposed to be ironic or just sloppy that Clark doesn't get to say goodbye to the love of his life (or even have 'Loana' make a final appearance) or his dead father, but gets a good sendoff for the son he never had and has only apparently known for a few minutes.
Overall, this story has some interest as a visualization/animation of the original Moore/Gibbons story, and it's an okay Justice League story, but as just a good story it kinda falls flat.