Dawson Casting: If you do the math, Mako, Raleigh, and Chuck are quite a bit younger than their actors.
Defictionalization: Two months after this film's release, tropical storm Haiyan struck through the central islands of the Philippines. What has that got to do with this movie? It coincidentally is the very first Category 5 Storm ever to make landfall in the actual Pacific Rim. Some Philippine editorials even alluded to the film.
Doing It for the Art: The sheer amount of detail that went into the concept and production of the film shows just how much love Guillermo put into it.
Fake Nationality: The Russian pilots, Aleksis and Sasha Kaidanovsky of the Russian Jaeger, Cherno Alpha are played by the very Canadian Robert Maillet and Heather Doersken. The Hansen father-son duo of Herc Hansen and Chuck Hansen are played by Maxi Martini and Robert Kazinsky who are an American and a Brit respectively. As are the two actors portraying the German scientists Newton Geiszler and Herman Gottlieb—Charlie Day is American and Burn Gorman is British (but was born in the US). Clifton Collins, an American actor of Mexican and German descent, plays the Chinese-Peruvian Tendo Choi. The Wei Tang triplets are played by the Vietnamese Luu triplets. And, of course, the British Charlie Hunnam as the American Raleigh Becket.
Also, Chau is obviously American, not Chinese. Lampshaded when he says he just mashed up the names of his "favorite historical figure and second-favorite Szechuan restaurant in Brooklyn."
Spanish audiences will recognize Santiago Segura, famous Spaniard comedian and film director, as one of Hannibal Chau's henchmen (the half-bald guy with the sunglasses that greets Dr. Newt at the entrance and later gets supposedly eaten by the kaiju baby). He is friends with Guillermo del Toro, and has had cameos in most of his movies.
The female sounding AI for the Jaegers is none other than GLaDOSherself. It's actually GLaDOS in all but name, the exact same process was applied to Ellen McLain's voice in the trailer. The final version has the voice altered a bit, but she's still recognizable at points.
The Japanese dub is basically a reunion of many big names who worked in the Mecha genre, namely Gundam and Transformers:
Viral Marketing: A viral site titled Pan Pacific Defense Corps was set up showing map covering blueprints of Jaegers from around the world and a video showing a kaiju attack. A later update allows you to sign up as a member and take a quiz for your role in the PPDC.
Writer Travis Beacham came up with the idea that Rinko Kikuchi's character Mako Mori was going to speak entirely in Japanese for the first half of the movie and her partner Raleigh wouldn't understand her. Then when they finally got their connection calibrated in the Jaeger he would hear her in English. Despite him seeing it as a good idea, he realised it would have been a chore to have the two leads speaking in different languages for the first half of the movie.
The aliens were originally written as full out Abusive Precursors. They created our universe to escape the entropic death of their own.
One of the earlier outlines had less diversity, with the Russian, Chinese and Australians swapped out for more Japanese pilots and the Shatterdome being in Japan. Also, Knifehead's death was going to be more akin to Shinji blacking out and Eva 01 going berserk
Crimson Typhoon was originally going to have four arms and be piloted by quadruplets, but it was already hard enough to find the Chinese triplets who were cast in the role so Typhoon's arms were reduced to three. In another draft, they were all female.
In the original draft of the film's script, Tacit Ronin is the only named active Jaeger in service alongside Gipsy Danger. Its pilots, Duc Jessop and Kaori Koyamada, developed a connection with Raleigh and Mako, going from uncertainty to camaraderie. It also had a laser capable of cleaving even a Category 5 Kaiju in half, and faced three of them in the original finale.
The Matador Fury was cut from the film due to time concerns.
Less important in the scheme of things but Raiju's inner head was more snake-like and he gave a bit more trouble in the novelization. Concept art of this longer and less horse shaped head is at the PR wiki.