Problem with the twist is that fake Mandarin was so damn intimidating and different from other MCU villains that losing him was a gut punch. While the idea of a fake terrorist is brilliant and raised some interesting political themes .
The twist caught the blunt of the internet doing its over reaction thing but the "problems" with the movie wasn't the twist.
A movie called Iron Man but Tony was out of his suit a HUGE chunk and honestly watching MacGuyver Tony Stark isn't my idea of interesting. And the times he was in the armor featured an armor that sucked without any weapons.
PTSD plot line was thrown away as quickly as it came. And never touched on again. It was a great idea and concluded in an unsatisfactory way.
All those cool armors at the end and they were used for 12 seconds a piece and destroyed like paper mache.
Oh no pepper is dead, oh look she's not dead, oh look she has super powers and his gonna do some weird kick missile thign to kill the bad guy, oh no she still has exremis and might die, well nvm Tony Stark can figure out it off panel!
The conclusion of the movie was just stupid. Wrapped out pepper stuck with extremis, destorying all the suits, arc reactor in chest in... 45 seconds of montage. And none of it stuck anyway.
If you wanted to have an "iron man story" where you go from Iron Man to IM2 to Avengers to IM3 and this concluded the character as he retired, teh movie works a lot better lol