Here's my opinion:
Mandarin is my only big problem with this movie, he was massively underused. He could've been Heath Ledger/Joker of the Iron Man films, in the beginning he was a dark, grim, cold and seemingly emotionless terrorist who wanted revenge on American arrogance, and he was also very mystic and it was great.
BUT then, later we found out that he's just a drunk idiot called Trevor.
Think about it this way, imagine if you saw Dark Knight. Batman has to face Joker's threats, but near the end he finally finds the Joker's hide out, he finds out that there is no Joker and it's just a dumb a*** named Bob.
Hardcore Batfans, how would you feel? Well, that's how most Marvel fans feel.
Other than that, I thoroughly enjoyed everything else. I would say its a film on par with say TDKR or TASM, good not great.