Great visuals, unappealing characters; ultimately very disappointing
The one and only thing I'll give this film is the spectacular visuals. The backgrounds, the settings, the colors, all were splendid.
The character design was very mixed.
The all-too-few human characters were appealing enough visually, in a traditional style rather than in the blocky form that blighted so many later Disney 2D features.
But the aliens were an ugly lot, right down to the cyborg. Such visually unappealing designs may indeed indicate the alienness of the world, but it doesn't make the film any more pleasant to look at. The villainous spider-like creature was actually one of the better ones. The various blob-and-tentacle beings were just too revolting to enjoy seeing in any capacity.
Long John Silver was so grotesque in his facial features and body that later, when the film gave him his redemption arc, it was still hard to like him in any capacity. This undercut the emotional power of the film.
The professor character and the female captain were also ungainly sights, their raccoon-like features making them less sympathetic than they should have been.
The female captain was especially off-putting. Couldn't stand her, hated every scene she was in. (And I hate to be particular, but that facial mole -- ugh! Reminds me of that Uncle Buck line: "Here's a quarter. Get a rat to gnaw that thing off your face.") I have no idea why the female captain was in the film at all, except for politically correct tokenism.
The first mate was a much more interesting character and should have been the captain and had more scenes. Predictably, the film killed off this character, one of the few decent ones.
The mother was a very sympathetic character -- indeed, the most sympathetic character in the film. And what happens? After the opening, she's gone.
Jim was at best a so-so character, another pseudo-rebel-type, an Anakin Skywalker without even Anakin's darkness, so ultimately just an angsty teen. Rather boring, actually, which one could unfortunately say of the film as a whole.
In fact, the feeling of the film as a whole was very Star-Wars-prequels, but without the good parts of Star Wars.
And the ultimate quest? For loot? Well, it's a goal of sorts, I suppose, but not exactly the noblest dream in the universe. Something crassy material about it.
I think Roger Ebert made the observation that it would have been much, much better as a traditional period adaptation of Treasure Island. As it was, this half-historical, half-modern steampunk look didn't ultimately work (though, as I said, it did provide some nice visuals).
Ultimately, a lack of sympathetic characters did in this film, I think, as did the unhappy steampunk past-present collision of identities. It just didn't have any of the great heartwarming moments that even lesser Disney films possess.
Of Disney's sci-fi forays, Atlantis (though its human-character design was much poorer), is the better film, though it suffers from a dreadful last act.