It is a movie about a giant monsters called Kaiju (Japanese for Giant Monster) emerging from an interdimensional rift and terrorizing the planet. To combat them giant robots called Jaegers were built. The Jaegers are too complex for a single individual so two have to work together to pilot them by using a mind melding technology known as drifting.
Why it good?
Well look at what you consider the best of the OSR. I bet you consider it good because it bring something new but at the same time it is thoroughly meshed in the tropes and mechanics of classic D&D. Without being overly wordy or pretentious.
That what exactly Pacific Rim feels like. Everything you like about the old Saturday Afternoon giant monster films is in here but better. There is a sense of a deeper background but they don't throw it in your face or waste a lot of time explaining it. The characters have depth and their issues feel natural. The actors do a good job.
Of course the Giant Monsters are awesome.
Don't get me wrong there is no exploration of deep truths in this movie. And there are a few issues, too much of it is in dark lighting, along one obvious technobabble flaw. But they are minor compared to shear awesomness of seeing giant robots, smashing giant robots.
It just fun. So stop reading this and go see it.
P.S. I were the Battletech guys, I would do a cut down version of their BT mechanics, figure out how to represent the monsters with a SSD add mechanics and get a license pronto. Or hell just make up their own monsters and release the game.