Let me start off the subject by saying that for those of you that already find this ridiculous as it pits one of the best MCU movies against one that's seemingly the literal opposite of it, I'm just pointing out some near similarities between these two movies and what Iron Man 2 might have been if it had undertaken a Winter Soldier attitude or a similar one of its own.
1. New sidekicks: After years of military experience, James Rhodes finally dons one of Tony's armor, later known as the War Machine. He eventually uses it to defend himself and Stark from the incoming drones and Whiplash himself, best fitting that Tony's lifelong ally now has a superhero counterpart, while Captain America runs past Sam Wilson in the streets of Washington D.C. after the cataclysmic events in The Avengers. Sam later reveals that he too was part of the military, and suits up to face the impending threat of the new HYDRA that threatens the safety of millions. While Iron Man 2 did have a better history with its sidekick and also the potential to blow up major portions of a city, it failed to maintain the swift paced and sustainable direction ensured by the Russo(s) in Winter Soldier.
2. Black Widow's sequel appearance: After hearing about Black Widow being cast in the Captain America sequel, I was of the impression that she was being used as a character for sequels and Avengers' movies. While she made a fairly astute appearance in Iron Man 2 which Tony regarded as "Mindblowingly duplicitous", she does have a bolder movie history this time round and starts to trust Cap after being on his side in The Avengers, and is of vastly more significance than before. Alright, that's debatable. I personally felt she was the real hero in The Avengers. Why? Cause she knew there was something awkward about the energy source of the portal, sure she borrowed a lot of hints from Dr. Erik Selving but even Thor couldn't figure that out. Iron Man 2 would have been part of this consideration, had Black Widow received a tad bit more time for her onslaught performances.
3. Nick Fury: Apparently showing up for a cup of coffee was fair in a manner of speaking, given that the charismatic entrance with the doughnut worked seemingly well, but was he of much significance otherwise as a character in action? I think not, which is where Captain America: The Winter Soldier does the real talking, you know shit just got serious when Nick Fury keeps both eyes open!
4. The ugly truth now revealed: Captain America: The Winter Soldier was the best thing to happen to the MCU after the Avengers, which was when most people had actually warmed up to the universe in general. I'm betting there were millions who pretty much skipped every solo movie and just headed to The Avengers. Now that things are in place, it's about time they get dirty. Hydra's evolution from the First Avenger works pretty well and adds a ton load of suspense to the climax. Even if Iron Man 2 had wanted to try, it would have had to wait for an Avengers movie to warm things up, unless of course, if Jon Favreau had pulled it off just like Iron Man.
5. Senator Stern: You either liked seeing him contradicted in irony in Iron Man 2 or just couldn't care all that much about him, but admit it, his appearance in Captain America: The Winter Soldier worked out well because he was already introduced once, throw in Sitwell too. A counter-argument could be made with the "All hail the king" one shot featuring a few underwhelming Iron Man villains that are getting done right, well...soon enough.
6. The timing: Let's take into consideration two scenes that are arguably the same. For instance here, witnessing a similar shot when Whiplash flipped Tony's race car or when Winter Soldier took out Nick Fury's armored car. While people complain an awful lot about Iron Man 2's setting, that is, how Whiplash even knew that Tony was going to be racing all of a sudden, it is notable that Winter Soldier might not have tracked down Fury's path all of a sudden either. That doesn't change the undeniable fact that he might have figured it out somehow, I take it he's like a ghost story as mentioned by Black Widow or was instructed step by step.
7. The Agents Of S.H.I.E.L.D. crossover! Season 1, episode 17 if I'm not mistaken. Yet another plus point that shells Iron Man 2's potential.
Alright, I'll still have to admit there were a few places where Iron Man 2 stood out as a considerably better sequel, like for instance, the introduction with the badass Whiplash theme as he constructs his suit after the movie picks up where 2008's Iron Man left off. But when you really think about it, it was obviously more flawed and lacked the quality of its predecessor. The only thing Winter Soldier might have lacked would be a charismatic set of sequences in a delightful song such as "The Star Spangled Man With A Plan", but it never did need one, did it?