Before I get started, I know Marvel Studios didn't have a full proof plan, starting their cinematic universe, so I know if they did, they may have done things much differently. I'm just stating why I feel that if Captain
America TFA was the first film released in the MCU, it would have caused less confusion, and possibly brought in a bigger audience for all of the Phase 1 films.
I don't know about you, but around the time Avengers trailers were coming out, I knew fans of comics and, of course, non comicbook fans that didn't know Iron Man, The Incredible Hulk, Thor, or Captain America TFA were all set in the same universe. If Cap's film came out first, with the same tagline "The First Avenger" I believe less fans would have been confused. I can't say the same for the non comicbook fans and the general audience, but there is little hope for them anyways. Fans of Marvel comics know who the Avengers are, and with the first film for Marvel Studios having the tagline "The First Avenger" many, if not all, would have known that they could potentially see an Avengers film in the future, and any future Marvel Studios film, could be leading up to it.
If I am right, and more fans of Marvel comics or comics in general, are aware of the potential Avengers film, the ones that would have skipped out on watching, Iron Man, Iron Man 2, The Incredible Hulk, and/or Thor, would have probably got themselves to go see these films, so they wouldn't miss any piece of the puzzle that is Phase 1. This would have brought in more cash for Marvel Studios.
Now I know, moving Cap's film to the beginning of Phase 1, would cause some problems, and here is how I would fix them. What about the Avengers promo after the credits? Easy, just add it to the end of Thor, which actually would make more sense. But How? And Thor already has and end credit scene! True, but Thor's end credit scene could be moved to the middle, and it makes more sense, because the events of Thor leads right into the Avengers film. If you move Thor's end credit scene to a mid credit scene, wouldn't more people leave, thinking there wouldn't be another scene? Yes, but if you end that scene with the text "Stay till the end, if you want to see more" or something of the variation, then you won't have that problem. Cap woke up at the end of his film, with it being first, wouldn't it screw with the MCU's continuity? Not really, even though Joss Whedon orginally had a scene, in the Avengers, where Cap is being defrosted, it ended up turned more into a flashback sequence, which it left how long Cap been defrosted unknown. Also it was never specified when Cap woke up in his film either; the closes thing we got to a date was Nick Fury telling him, he's been asleep for 70 years, which was just rounded to that.
Some other advantages of Cap's film being first, they could have done an end credit scene, teasing Iron Man. The end credit scene of Iron Man would have brought more clarity to the fact that Marvel Studios was building a cinematic universe, leading to an Avengers film. Cap's shield being teased in both Iron Man films, would furthered the idea even more. With the Tesseract being a weapon used in Cap's film, and also being a weapon used in The Avengers, putting Cap's film first would have brought the secret of the Tesseract full circle.
[Update] I feel this need to be added, but I like the way the timeline(by timeline I mean ordering of the film's releases, and not the timeframe in which the films take place) has been set up in Phase One. I'm am not saying Marvel Studios made a huge mistake by doing things the way they did, but that if Iron Man wasn't the life line of the MCU(if it failed the MCU would not exist) then I would of thought further ahead, since I would have the ability to do, and I would have made Captain America TFA the first film in MCU's Phase One. I also feel that if they knew exactly where they were going with the MCU to begin with, they might have made the same decision.