I'll confine my answer to the Phase One movies, which is the time period I was talking about. I think the biggest thing is the everyman aspect (OK, the biggest thing is Clark Gregg, who's just so personable and enjoyable). I do agree he started small and almost inconsequentially in Iron Man (his whole point was pretty much for that SHIELD joke). I liked him because he was a tease of the bigger universe. Thor was the same way. He was almost a villain if you think about it, but his moments of reaching out to Thor was nice. Then came Something Funny Happened on the Way to Thor's Hammer. He's a hero without costume or powers, but he still saves the day in awesome fashion (that apparently used up their budget for their first two one shots). Then came Avengers and the scene with Loki. When I was in theaters, the whole crowd felt emotional at Coulson's death. I suspect we weren't alone in that.