The team is more of an elite military unit than a group of noble superheroes, groomed to fight the controversial War on Terror. In-Universe, views are conflicting: people in America see them as Eagleland Type 1, but for foreign powers they are a Type 2.
Betty Ross/Ultimate She-Hulk was created as a sweet girl that fell in love with Bruce Banner, in stark contrast with her father, "Thunderbolt" Ross, an always-angry General Ripper. The Ultimates take preference for her Military Brat aspect instead and make her an Alpha Bitch.
Captain America, unlike his mainstream counterpart (who is the example for Incorruptible Pure Pureness most of the time), isn't just a man from the 1940s, he's abrasively rude and holds many of the same bigoted views prevalent at the time (and some that weren't). He also lacks the main Cap's diplomatic side, which was clear when he was elected president: he told state governors to solve their disputes between themselves instead of asking for his mediation, and when a state tried to secede from the Union he forced it to stay in it... the manliest way possible. He regularly dismembers and kills his opponents (as a cinematic trope usually employed in Ultimate Marvel is that Superhero Movie Villains Die), and unlike his 616 counterpart he won't hesitate to Kick Them While They Are Down.
The Incredible Hulk is a Psychopathic Manchild with cannibalistic tendencies whose brain basically revolves around his desires for killing, eating, and having sex with whatever he chooses, without any form of moral or mental restraints. He's explicitly not part of the Ultimates, Banner is. Hulk's treated as more of a living weapon than a teammate, especially after his rampage across Manhattan that killed over 800 civilians. Banner's not out of the woods either, as he intentionally triggered that rampage after Betty dumped him and tried to play it off as a Zero-Approval Gambit. And Betty actually takes him back because of it.
Hank Pym in the main universe has his issues and hit his wife once in a fit of rage, but never escalated beyond that and has spent years trying to make up for it. Ultimate Hank is a repeated abuser and once attacked her with an army of ants while she was shrunken down (after he gassed her with bug spray). He put her into a coma with this stunt.
Hawkeye is a cold black-ops agent, instead of the jokester of the main universe. It only gets worse when his family dies, when he becomes a Death Seeker who antagonizes the rest of his team.
Pyro initially started off as a case of Adaptational Heroism, helping other people and joining the X-Men. Then came The Ultimates 3, where he infamously did something that got his hands cut off and Mastermind beheaded, and would appall even the classic Pyro — suggesting he and Mastermind rape Valkyrie.
As opposed to the classic Betty Brant, this Betty was a general jackass to people, including sleeping with Kraven, throwing around Ned Leeds's drinking problem to get a story she wanted and making bets about Ben Urich's disappearance. And during Miles Morales's time, she died before she could "out" Jefferson Morales as the second Spider-Man, not caring about how it'd affect Rio or Miles.
In the mainstream comics, Flash Thompson started as The Bully, whose only purpose was to make Peter's life at school miserable. Eventually he got a more sympathetic characterization, joined the army, and even became a superhero himself. Ultimate Flash Thompson, on the other hand, stayed in the original characterization and never grew beyond it. Also unlike his mainstream counterpart, this Flash isn't a huge fan of Spider-Man; in fact, he hates superpowered beings, especially mutants.
Downplayed with Peter Parker himself, to the point where he's one of the very few truly good and decent heroes in the Ultimate universe. He does have some moments of this, however, especially early on in the series before the Comes Great Responsibility lesson truly sinks in. When he meets Miles Morales, his reaction is the exact opposite of his mainstream counterpart; while 616 Peter acknowledges Miles as a worthy successor, 1610 Peter snaps at him that he never should have become Spider-Man, beats him unconscious, and takes his web shooters back. He does eventually come to view Miles as a worthy successor as well, but it takes a bit longer for him to do so than 616 Peter.
Professor X compared his love for his son to that of a pet owner and their pet, and was willing to create a drug made from Wolverine's DNA. The main continuity Xavier has strict boundaries about not reading minds without permission or unless it is an emergency. The Ultimate version in his introductory scene casually mentions he reads minds for fun, specifically that he likes to prowl through the minds of his favorite authors and see what interesting ideas they come up with that never end up published.
Because he was among Weapon X's victims, Nightcrawler had a brief psychotic break where he was a Stalker with a Crush to Dazzler and homophobic towards Colossus, though he later made amends.
Magneto was outright genocidal, and lacked the 616 version's tragic Holocaust backstory to boot. Also, while the mainstream Magneto won't be winning any Father of the Year awards anytime soon, Ultimate Magneto took this to another level, constantly berating Quicksilver and humiliating him in front of the other Brotherhood members. He once even asked Cyclops to refer to him as "Father" in front of Quicksilver just to further crush the boy's spirit.
Dazzler is openly rebellious and prone to swearing, since this version of her has more of a Punk Rock theme than Disco.
Matt Murdock starts out as a decent guy in the prequel miniseries, but by the time he appears in Ultimate Spiderman he's definitely Taken a Few Levels In Jerkass. While he does mean well and will ultimately do the right thing, he's often hostile, abrasive, rude and downright petty — and also more lax when it comes to breaking the Thou Shalt Not Kill rule.
