The biggest thing to say about the movie that launched Marvel Studios is that it felt genuine and natural.
Getting Robert Downey Jnr as Tony Stark/Iron Man, you could not get better casting. When you have an actor that a role is tailor made for and can be an extension of his personality as opposed to an actor having to learn to become a role makes all the difference. RDJ brought his wit and charisma as the CEO Stark Industries and the man that got inspired to create the first Iron Man Suit after his capture in Afghanistan.
Considering the budget that Marvel had to work with when compared to movies that would follow with the likes of Thor, Captain America and the Avengers; Iron Man did very well with what they had to work with which was $140 Million. That's still a lot of money but for movies today that have a large use of special & visual effects, 140 M is almost a medium budget.
Gywenth Paltrow is very believeable as Pepper Potts. For someone that was as forthright, outspoken & unpredictable as Tony Stark, you needed that opposite that could call him out as and where applicable.
Jeff Bridges. One of the best actors in the business and fits in nicely as the stoic Obidian Stane aka the Iron Monger is at times Tony's voice of reason for his unforeseen decision to retract from the weapons business.
I think what makes this movie so rewatchable is that it came at a time when movie fans, dare say superhero fans were looking for something fresh that would reinvigorate not only the marvel movie franchise, but comic book movies in general.