Complete Monster: Commander Lyle Tiberius Rourke is a tomb-robbing mercenary (or, as he insists, "adventure capitalist") solely in the Atlantis operation for profit. The leader of the expedition sent to find Atlantis, Rourke maintains a gruff, militaristic indifference to the lives of those lost to the Leviathan and upon seeing the Heart of Atlantis, plans to confiscate it and sell it for double the money he'd receive. When told that taking the Heart of Atlantis will kill every Atlantean, Rourke proudly plans to triple the price for it. When Milo refuses to cooperate, Rourke threatens to shoot Princess Kida and shortly thereafter murders her father, never dropping his friendly façade. Rourke was concerned for little else but himself and the potential profit he could reap from Atlantis, to the point where he tosses Helga off a blimp to her death for a minor benefit to his escape, and was greedy enough in the end to make his entire party turn against him. With his willingness to commit genocide against an entire civilization just to profit of their deaths, and many more heinous crimes, Rourke is indeed a despicable piece of work.
Genius Bonus: They did some research and then deliberately played with it.
Ichthyologists and paleontologists in the audience had to laugh at Whitmore's aquarium, which contained coelacanths (a 65-million-year-old species that was thought long extinct until its rediscovery in 1938.) Probably meant to show, in the most obscure possible way, that Whitmire's been exploring the world long enough to find all sorts of hidden things.
Cookie shows off a map on his belly showing "all 38 states." There were 38 states from 1876 to 1889 (meaning he got the tattoo sometime between age 33 and 47.) He seems to think there are still only 38 states, but then, he's not all there.
While abandoning the main characters in Atlantis, Rourke mutters, "P. T. Barnum was right." Mere gibberish unless you know Barnum's most famous (and probably apocryphal) saying, There's a sucker born every minute.
Harsher in Hindsight: The King's last moments are spent coughing violently. Leonard Nimoy would die in 2015 due to complications from being a chronic smoker for much of his life.
Moral Event Horizon: Rourke crosses it and keeps on going when he punches the elderly King of Atlantis so hard it causes internal bleeding, eventually leading to his death. Dr. Sweet drops out without hesitation when he sees this happen and while the other main crew members don't, Audrey and Vinny at least are shown to be disconcerted and repulsed.
This fully deserves its place in the YMMV section, because on the contrary, some people think that it's one of the few cases of a very good spin-off TV series that they are sad wasn't made. Of course, this requires one to watch the movie as it was conceived — a package of TV series episodes — and from that point actually it is a pretty good TV series.
So Okay, It's Average: The general consensus seems to be that it has a lot of good points, but a lot of bad ones too with each kind of balancing the other out.
Theiss Titillation Theory: Kida's Atlantean clothing bares much skin. Okay, so that's an Atlantean thing. Helga, on the other hand, wears some things with straps. Often, there is only one firmly on a shoulder at at time...
They Wasted a Perfectly Good Character: Lots of effort is devoted early on to establishing this huge herd of interesting supporting cast members, who then spend the rest of the movie tripping over each other for screen time to the point where it feels like all of them got shortchanged. If the proposed TV series had materialized (see "Sequelitis" above), they would have each finally gotten a chance to shine.
Vanilla Protagonist: Milo, especially compared to the above mentioned herd of quirky supporting characters. He's not exactly a Flat Character but he's not particularly interesting either. Just a nice, normal nerd with a talent for linguistics who gets to go on an adventure.
Vindicated by History: The film was neither a huge financial or critical hit, and perhaps received one of the most lukewarm receptions of any Disney film at the time. Many questioned Disney on its choice to pursue an action-oriented animated film, one that was designed for an older audience in mind and resorted to the extensive use of CGI when traditional animation had begun to lose favor at Disney. However, these days it is becoming a significant cult classic with more and more viewers taking a closer look at the film's merits. Those who enjoy it tend to appreciate its comic book style animation and its many adventurous elements, while many in its supporting cast have grown to be fan favorites among some, especially Audrey, Kida and Vinny in particular.
Visual Effects of Awesome: The film has very good animation on the whole, but the animation on the Leviathan is amazing. Similarly, any scene with Kida and the Crystal is bound to have this.