In the Marvel Cinematic Universe we have seen lots of character changes. Some are neccesarry, some are good, some are bad. These are the character changes that I found to be the largest.
1. Iron Patriot.
The Iron Patriot in the comics is Norman Osborn. You know the green goblin? Well pretty much he gets an Iron Man suit and paints it then makes the Dark Avengers. Its a small comic line and isn't the most known of characters. Now the changes in Iron Man 3 to the character was that they just painted War Machine. This was a useless change because War Machine would have been just as cool in Iron Man 3, however its not a big deal. Even if they owned Norman Osborn, Disney was never going to make a comic movie about this story. So it was just a small Nod to the character that doesn't really hurt anything.
2. Justin Hammer
In the comics Justin Hammer is old. Probably around his 60s, and he is scary. Much more evil seeming then in Iron Man 2. Also he is a lot smarter than he is in Iron Man 2. He does some crazy crap in the comics, much along the lines of his drones. In the movie he didn't even make those though. Now age isn't really all that big of an issue to me. He did become scary at parts, the scene where he freaks out on whiplash saying they had a contract was a pretty good scene. However I think most of us wish he would have been a tad smarter throughout the movie. Who knows we may see him later and he will have grown up a tad. But if that's all we see of Hammer its not the best, but its not really all that bad.
3. FirePower (Spoilers)
For those of you who don't know, Firepower was in Iron Man 3 as the guy that blew up and injured Happy. In the comics he plays a guy who is in a huge suit (Kind of like the Iron Monger). It holds amazing weapons including missiles and a small tactical Nuke. That's some fire power. Now the change just turned him into the guy who pilots the suit, and he just blows up. The nod to him is his dog tags. So pretty much all this did was tell use that we won't be seeing a Tactical Nuke launched off of a suit. If the totally redid the story, we could have had a potentially sweet scene with him against stark, but its no big deal. Fire Power isn't well known and its nice the put a name on the random guy blowing up.
4. ColdBlood (Spoilers)
Cold Blood is a lot like Firepower in the way that they pretty much named someone. In the comics ColdBlood is an assassin, agent, and cyborg for SHIELD. The guy is only in like 2 comic lines if I'm not mistaken, (Like firepower) but he's a good guy. He is simply an assassin who doesn't do much. In IM3 He plays Killian's right hand man. He steals the Iron Patriot suit, kills a couple people, and eventually dies. Again its like Firepower, with a totally different story he could have played a cool assassin. He was again just a character that they decided to give a name in the movie. No big deal.
5. JARVIS
In the movies JARVIS is Tony Stark's AI. It can control his suits, build them, and just about everything he wants it to. In the comics Edwin Jarvis is Tony's Butler. He serves a lot like Alfred does to Batman. There is the problem. You can't have Edwin Jarvis because lots of people would have just said it was Alfred but not as good (Lets be honest nobody could top Michael Cain that I know of). Sure you could have tweaked him but I like just having the AI. However they could have made more of a nod to Edwin. My favorite idea would just to say he names JARVIS after his old butler. It would make sense, it would take a 5 second clip, and there you have it. We know Edwin has died and has an AI named after him. But alas we just have the AI which is still pretty good.
6. Hulk Origins
The original Hulk origins are kinda crazy, first he's gray and then... well... In the comics Banner saves a teenagers life. The teen drives into a Gamma bomb testing zone and banner saves him. when the bomb goes off banner gets turned into the hulk and is a gray hulk. They kinda cure him later in the comics, it comes back, its weird. In the movie that take the road that the old TV show took. He does it to himself, however in the movie it was because he was trying to find the super soldier serum. Here's why I think it was a good change. It makes more sense to have them looking for the serum because it would be a much more realistic (if there is such a thing) way to find the hulk. If a gamma bomb goes off, you would die. Instead this is triggering his cells in order to try and make him stronger but it just had the side affects. Also it gives the military more motives to chase him because they want the super soldier serum. The Hulk hasn't been known for his origins as well as Batman or Spider man, so I think it was a good thing to sacrifice to make the movie universe fit more.
7. Bucky
In the comics, Bucky is a teenager in the war. His father has died, and he is a mascot for the camp he is at. He finds Captain America Changing into his suit and asks to be his partner. He gets trained, the army allows him to fight as a teenager to rally the young people and off they go to fight Nazis. He also "Dies" at the hands of Baron Zemo, not a Hydra agent. If they redid the script they could have had Bucky in his original origins story. But it would have been very hard to make it feel at all realistic. Also to have him die by Baron Zemo would have required a lot of changes to the movie. Mostly because Zemo deserves to be a main villain in a movie. So although it would have been cool to see the teenager killing people, it wouldn't be very realistic, and it wouldn't fit into a movie well.
8. HYDRA and the Red Skull
In the comics, Baron Strucker makes HYDRA. Also the Red Skull isn't the leader of HYDRA and is mainly a Nazi in the war (until later on). You saw the movie, you know that he leads HYDRA and ect. Now having the Red Skull the leader of HYDRA makes sense here's why. Red Skull would be boring if he wasn't in charge (in a movie). Also we needed HYDRA to get known so we can seem them later in the movie universe. Also there are hundreds of Nazi movies so we didn't need to seem them again. Now there could have been ways to have Strucker in the movie still. They could have easily had Strucker. One way would be when all the Nazi leaders show up and RS kills them, they could have just had one of them be the leader of HYDRA. Have RS be just the weapons maker or something, then have him kill Strucker and just say that he's in charge now. Sure it wouldn't do Strucker 100% justice, but it would be better than never having him in the movies. Oh well. Now in the comics the Red Skull is just wearing a mask, in the movie he uses the serum and it changes his head. This I don't like. They didn't have to change that part. They could have had him use the Cosmic cube to fight Captain America like in the comics. Instead they gave him a red fave and super powers. Sure it was easier to do, sure it worked. But I wish they hadn't made him over powered and had strucker.
9. Nick Fury and the Howling Commando's.
In the comics, Nick Fury is a super soldier like Captain America. Not as good of one but he is one. He also is a Member of the Howling Commando's. Now this was a huge missed chance in my opinion. The Howling commando's should have had a scene to tell people who they were. The characters where well known in the comics, why not let us know who they are without looking it up? When they where in the bar they could have just given their names, then we know who union Joe was, and also they could have had Nick in there. He didn't have to be anything special, maybe just introduce him by saying "I have some enhanced abilities" and then just go about the movie with him in their team. Doesn't have to be a big part but they could have had him. Its not like it really hurts anything not to have it, but it would have been cool to see the full nick fury. There are a lot of other teams like the invaders and such that they could have had during WWII but these wouldn't have been very possible to do. There are other super heroes that deserve their own movie before being thrown into a random movie just for the sake of having a team.
Lastly, I think you all know this one. (Spoilers)
10. The Mandarin
We all know who the Mandarin in. Chinese guy with 10 rings right? But in Iron Man 3, he is a white scientist named Adrian Killian. Only thing Chinese about him are some tats. His powers also come from the Extremes, not rings. This is a drastic change. They named the Mandarin differently, changes his race, and his power. He wasn't even the Mandarin. It was sad that we didn't get to see him in all his power. We all know this, but in a little bit of defense. They found the Mandarin to be racist, I disagree but if they didn't feel comfortable then that's their opinion. Now they still could have done the story (would have needed a totally different storyline of course but they could have) the first half of that movie was amazing. Tony was getting pushed (Hence giving his address to a terrorist), and wasn't being smart. He was getting emotional and couldn't really stop. Then the twist came and it kinda fell apart. It was still a good movie, but it wasn't nearly as good as the first half. Now they did ruin the Mandarin we loved, but lets be honest here. There are thousands of villains, the Mandarin isn't the most famous. Sure he's a good one but looking to the future. They ruin a villain its not to big of a deal. The MCU probably wasn't going to have need of him especially without another solo film. I disagree with it as much as you, but I am trying to give a different perspective as to why it wasn't to big deal. If they start making a habit of this character smashing, we may have an issue.
Sure there are some more minor tweaks, but these are probably the biggest character changes in the films besides appearances. Compared to other movies, they have stuck to the source materiel pretty well, its more of just missed chances to add something else in to give us something like Jarvis or Nick fury. Maybe your someone who just loves the changes and that's fine, I personally like staying closer to the comics.
Marvel's movie list for the next few years
Thor: The Dark world
Captain America: The Winter Soldier
Guardians of the Galaxy
The Avengers: Age of Ultron