A fun movie to watch. A movie that pays respect to one of Marvel's most successful comic book series since 1963. For full coverage of the movie, see
Full Review here.
Do not read if you haven't seen the movie:
Fun Facts about the Characters: Black Widow and Hawkeye were Iron Man villains in the original series before they became Avengers. In the Ultimates version, Black Widow was a double agent working to manipulate Stark and steal his secrets to the Soviets. Hawkeye was someone who is skilled with any weapon.
Thor has the strength to match the Hulk. In both versions, he has the ability to close worm holes and his hammer can easily penetrate common force fields (the hammer is also much stronger than any repulsor ray or impact from Hulk's fists). His power is not limited to lightning and strength, he commands all the elements. The Ultimate version of Thor may be slightly weaker than the original version (616 universe). Hulk was stronger than Thor in the Ultimate version.
Captain America's shield is made from an accidental mix of vibranium and adamatium (Wolverine's claws). His uniform was made from chain mail in the 616 version. The Ultimate version of Cap is much stronger than the 616 counterpart since he could take down Giant Man and topple the Hulk. The ultimate version of Cap can jump from a plane in a modest altitude without a parachute.
Loki was more of a trickster and manipulator in the Ultimate version. He opened a wormhole to bring Asgard's enemies to conquer the Earth (storm and Frost giants at 500-1000 feet in height, trolls and ice and fire demons). He also assembled a group to match the Avengers. In the 616 version, he had enough mastery over sorcery to take down 2 teams of the X-Men and Alpha Flight. He is indeed the reason that the Avengers banded together. In the Ulitmate version, the Avengers were a government financed project to assure the public that they have a plan against the mutant threat or any other threat.
Samuel L. Jackson's Nick Fury is derived from the Ultimate version. In the 616 version, he was a military colonel who had ties in Steve Rogers. He was kept alive with an anti-aging serum called "infinity'. The avenger Mockingbird now has both the super-soldier and infinity serums coursing through her veins.
Tony Stark is more closely derived from the Ultimate version. The way his armor is assembled and the manner he carries himself is also more closely similar to that version. In the 616 version, he is a much more cunning, moody and much more resourceful. Currently in the 616 version, he has a nanotech armor that comes from his skin. This armor is impervious to Magneto.
Hulk is definitely based on the Ultimate version since in the movie he was a product to try and replicate the super-soldier serum coursing through Captain America's veins. Though the coloring in the ultimate version is Grey, the movie opted to use the more familiar green color.
The story of the film is very loosely based on the Ultimates 2 storyline Gods and Monsters.

Sorry to bore, hope this helps!