Now that I actually have seen Pacific Rim, I can finally talk about one aspect of the movie that I'd been wondering about since first word about it started to leak: the way del Toro explains why things exist as they do in the film. Why send giant robots after giant monsters when you could just nuke them? (Well, for one, then we wouldn't have a movie — or if we did, it would be about seven minutes long.)
I noted before how too much of this can be just as bad for a movie as not enough, because if you give a fanboy mouse a plot cookie he's going to want a glass of plot milk to wash it down with. Not enough explanation and people are bewildered; too much of it and people get overwhelmed and bored. This goes double for SF&F, which more often than not require explanations to set limits and establish rules about how things can unfold. A movie where everything is possible is also one where nothing matters, so you don't want to go too crazy with any of that. But all too often you're forced into explaining everything.
Nobody was particularly bothered by the lack of rules about the Force in Star Wars, because it seemed tied deeply into what the characters were all about. Luke doesn't force-choke people because that's just not how he rolls, but he's not above a Jedi Mind Trick or three when the circumstances demand it. But then came Star Wars The Previous Generation in which we heard the word "midichlorians" and we all winced liked we'd been sliced with razors and rolled in lemon juice. We didn't want the presence or absence of the Force to become a dumb plot hinge. (Turns out there were far worse things to worry about, but one disaster at a time.)
Back to Mr. del Toro. What I did like about Pacific Rim was how one of the things that seemed arbitrary in the film, on an explanatory level — why two pilots are needed to run the Jägers — turns out to be a thematic element in the long run. The story's loaded with instances of two combining their powers to become one (a common theme in mecha stories, actually), and they all end up having thematic weight. This is far from being the only way to make what seems like an arbitrary ingredient into a valid one, but it's one of the better ways to do it.
I cannot now remember where I heard this, but I once got wind of how, in Japan, there's what has been described in movies or books as "Too Much Explanation Disease". You know this one: it's where the creators laboriously spell out every last painful little detail, out of the paralyzing terror that the audience won't understand what's going on and will bunk off before they get through Act I. SF and fantasy are particularly vulnerable to this, because the general assumption is that the mainstream viewer is a thumb-sucking bonehead and will hate whatever he doesn't understand.
This makes creating an actual science fiction movie a losing proposition, and so even a relatively brainy project like Inception has to be turned into a "heist movie". Everything below that becomes a generic action film. And everything below that ... um, don't go there.