Tonight at dinner Pam asked me, apropos of nothing, what I look for in a science fiction movie. I don't know what prompted the question, but it's a good one.
I told her that what I look for is ideas. A good science fiction movie has interesting ideas, and explores the implications of those ideas. A bad science fiction movie has no interesting ideas. Or worse, starts with an interesting idea and abandons it in favor of a generic action movie plot. Paycheck is a particularly egregious example of the latter, and there are countless others.
A good example of the former is Gattaca. By objective standards, it's not that great of a movie. I love it nonetheless, because the story is so thoughtful. I've seen it several times and would happily watch it again. The Rod Taylor version of The Time Machine is another wonderful movie about ideas, which I forgot to mention to Pam. As I've written here before, The Time Machine is one of my favorite science fiction movies, if not my very favorite. Which makes the stupidity of the Guy Pierce version all the more sad.
It's really not about the special effects for me. Effects have their purpose, they can be impressive, beautiful and/or exciting. When used well they can create a sense of immersing yourself in another world: The Lord of the Rings for example. But the effects have to serve the story, not the other way around. The Time Machine proved that a science fiction movie can be engaging without expensive special effects. (I know it was a long time ago, but I think the stop-motion effects in The Time Machine were not state of the art even for their time.) Gattaca goes even further; in fact I can't remember a single effect in the entire movie. There's a scene where Ethan Hawke longingly watches a spaceship take off, but I can't remember if you can see that it's a spaceship, or if it could have been actual footage of a plane overhead. Either way, the effect is clearly not the point of the scene.
On the other hand, I have no desire to see an sf movie with impressive effects and a weak story. Like, for example, Star Wars episodes 1, 2 or 3. Haven't seen them, don't really care if I ever do. I guess I've lost my tolerance for movies that rush from one explodo-fest set piece to the next. I want science fiction to give me something to think about.
So what do you look for in a science fiction movie?
Phantom Menace illustrates your point but in an odd way. It's not that Lucas doesn't have any ideas. Rather, he's got too many and no ability to get them to pull together into a coherent story so the movie becomes a series of FX driven set-pieces separated by incomprehensible talk shows trying to explain his elaborate backstory (too much plot getting in the way of the story). Plus, one of his biggest ideas (midchlorians, of however it's spelled) actually undercuts his big idea from the original trilogy. I just read someone ranting about this recently, in fact. If one's ability to use the force is determined (controlled?) by micro-organisms in the blood, then much of the personal quest/struggle for self-control aspects of the original trilogy seem to be undercut. At the very least, in terms of actually thinking about what your idea means (as opposed to just going "wow, that's a cool idea"), shouldn't the Sith (if not the Jedi) been engaged in massive genetics projects to engineer high midchlorian levels? Develop the midchlorian equivalent of steroids? When the Force is a psychic phenomenon that individuals tap into, it makes sense that you'd have to wait around for a Skywalker to show up. But if you know what causes Force sensitivity... maybe not so much
For me, it's about worldbuilding, which yields the immersion you're talking about. I can sometimes even overlook multiple lousy aspects of a film in other areas, if the immersive quality is really well-done. I will say, however, that when my brother and I first saw The Matrix in the theater, we were both too bowled over by bad writing AND acting to enjoy it to the normal extent. I did learn to like it later, but it took repeated viewings and peer pressure. ;)
I agree in the extreme with what you say about special effects. Successful worldbuilding doens't need to rely on them; several episodes of The Twilight Zone prove that, as well as numerous classics like your favorite, Time Machine.