The single biggest problem was that the actors couldn’t see the monsters they were fighting. In Jurassic Park, they addressed this issue by putting up a tall pole with two light-bulbs on it, to represent the T. Rex’s eyes. Now, the actors are all looking in a definite direction…and in the same direction…and focussing on something definite.
The scenes in Golden Voyage of Sinbad, where Sinbad fights the six-armed statue, is another example. In the long views, Sinbad’s cuts and parries don’t actually make any sense. He’s just waving his sword around. In the close-ups, where the actor is actually fighting against physical swords on poles, the fight makes much more sense. He can actually see what he’s parrying.
(This really fucked up “Sky Captain and the World of Tomorrow.” The actors’ actions had absolutely zero relationship to the things happening around them. The poor actor thrashing around on the floor, dodging the footfalls of the giant robots…could only dodge left and right at random, because she couldn’t see the robots’ feet. The two guys wrestling on the edge of a precipice never showed any awareness of their danger…because the actors couldn’t see the edge. Even a little set-building and prop-work could have fixed that!)
They did this in the Harryhausen movies, too – the technique wasn’t invented by the Spielberg film. There are production shots that show the poles and the plywood cutouts that the actors are reacting to.
For The Seventh Voyage of Sinbad, actor Kerwin Matthews practiced his sword fight moves repeatedly with a fensing master until he could do it exactly the same (No small feat, as he observed, when you have to stop your heavy sword as if a skeleton is parrying). His muscle memory allowed him to react properly to a nonexistent opponent.
They also did something similar for Who Framed Roger Rabbit, where they actually made 3D rubber replicas of Roger and the other characters for people to help them visualize the scenes.
Although I do look back fondly on Clash of the Titans it does suffer greatly from the R2D2 effect. Everything that they wanted to appeal to kids had to have a cute robot sidekick. Just like the daggit in BSG or Twiki in Buck Rogers, every time I see that damn owl I want to stab someone with a fork.
While the original Clash of the Titans is part of my childhood, I can’t share the joy with Kizarvexilla (who is now 14). One look at Perseus’s coif and she collapses into paroxysms of mirth, during which the only intelligible words she can say are “80s hair.”