Pacific Rim is not an original film--and it is not trying to be. The big premise--giant mecha fight giant monsters--is marked from the opening prologue/voice-over as derivative: the monsters are "kaiju," just like Godzilla and all the giant monsters in Japanese movies (which, truth be told, probably have some connection with 1950s American B-movies); and the giant mecha are "jaeger," which, frankly, is a word I first learned from Battletech.
On top of that, the character arc isn't original: we have the brash kid who doesn't listen to orders and has a prior trauma to overcome; we have the stern father-figure military commander who gives the requisite speeches (don't touch me; we're going to save the world); we have the beloved daughter-figure who just wants to prove herself. (Classic Freudian and a very common character-structure in SF: son-figure has to displace father-figure to love daughter-figure.) We have one over-talky mad scientist and one over-weird mad scientist. We have the brash young rival. We have A FREAKING DOG. (For reference for that, see everything ever made, but especially Cowboy Bebop.)
The plot, well, you know the drill: the alien invasion is getting worse and our only hope is to collapse the portal. (The most obvious and recent parallel is with the Avengers, which features another person in power armor seemingly sacrificing himself to deliver an atomic bomb into the enemy world.) There's occasional little jokes, including "giant robot fist crashes through building, taps desk only hard enough to make the Newton's cradle start tapping" and "let's check his pulse ::shoots up monster body:: no pulse."
And yet, it all kind of works. It didn't blow me away, but it kept my attention, and there were little details that I really liked. For instance, as many people have noted, some of the names here are capital R Ridic: Stacker Pentecost, Raleigh (like "rally," like "rally the troops") Becket, Hercules Hansen, etc. But then, when Hannibal Chau introduces himself, he notes that he took his name from his favorite historical figure and his second favorite Szechuan place in Brooklyn. And while we focus on the giant robots, there's a real industrial, lived-in quality to these spaces. (There's also a tiny shout-out to pollution and climate change: the aliens are invading because we've practically terraformed the place for them.)
So while it isn't really an original film, it does well what it sets out to do, which is mostly be a shout-out to a sort of 80s anime/cartoon show, complete with delightful international stereotypes on the team. I think we all want to know the real story behind the Russian death-metal pilots. Similarly, the idea of "drift"--the combination of minds that lets two pilots safely control one of the jaegers--isn't really original, but it allows Guillermo Del Toro to invade people's minds and give little glimpses of their character-forming moments, either as brief moment (crazy doctor getting tattoos) or as long sequence (love interest Mako Mori's childhood catastrophe facing a kaiju). In a way, that combination--derivative but interesting--reminds me of Cameron's Avatar.
But then Avatar did a lot better. Currently, this movie has made back a little less than half of its $190 million, which means that it will probably break even at some point, but probably not spawn a series of sequels. I have a couple thoughts about why it has performed OK, and perhaps the first that should be mentioned is that it's only playing four times a day here in San Angelo. (For comparison, Grown Ups 2 plays nine times a day.) Is that because it's long (2:11) or because no one thought people would be interested? Another problem may be that the buy-in cost, the gimme, is just too high for people: if giant monsters attack, are we really going to build giant robots?