If I had to say why, it might have been:
- Better villains: Making Jeff Bridges a bad guy was kinda stupid. And suddenly trying to kill Tony Stark at the end of part one made no sense for his character. The whole ending for Iron Man 1 was probably the worst part of the whole move. On the other hand, Iron Man 3 has funny villains with awesome powers. In other words, these guys don't frown all the time and ★■◆● about things they hate.
- More action: Iron Man 1 was really good, but was mostly about developing the Iron Man suit. Iron Man 3 seems to have more action and better ending battle.
- More Pepper Pots: She plays a bigger role and is far more important in this movie than in any of the others in the series.
- Sad Tony Stark: he has mental issues like in Iron Man 2, but they don't drag the movie, like they did in Iron Man 2. In Iron Man 3 they always find ways to fit humor into Tony's suffering.