Rewatching the Marvel films in order, so:
Iron Man (2008)
It's fair to say the film has dated - the CGI is no longer cutting-edge, which is reasonable after seven years - but not truly aged, beyond a reference to MySpace. The weak spot remains the final fight, and I think the reason is because Tony Stark essentially becomes humourless during the whole scene. Yes, he makes a few gags, but they're typical action-film quips, not RDJ-style gags. I don't quite know how you'd fix such things, but it's inevitably off tonally as a consequence. Phil Coulson, on the other hand, is in this film less than you'd think, showing up about five times, with at least the first three occasions being largely inconsequential in themselves; it hard not to feel that, for all the bitching the SHIELD parts received in subsequent films, it's there the character's reputation truly built.
Reportedly, the post-credits scene was written by Brian Michael Bendis, and edited down from three pages of dialogue. Needless to say, either of these facts is implausible without the other.
FUN FACT: the word "problematic" shows up in this film in pre-Tumblr usage, on this occasion to refer to hull pressurisation.