Kind of spoilerish, but if you've seen the movie, read on!
Kind of spoilerish but feel free to read after you've seen the movie, if you already have then read on...
Iron Man 3 is Tony Stark's search for the answer to Captain America's question back in The Avengers: "Big man in a suit of armor. Take that away -- what are you?"
Tony had been holding onto his arc reactor chest piece because he thought it defined who he was as Iron Man, which is also why he spent a lot of sleepless nights building and building more armors.
In the end of IM3, with the help of the Extremis formula on Pepper, Tony could use it and also finally let his arc reactor go because he finally understood that Iron Man is the man who "built" the armor (The Mechanic) and not the armor that wears the man.
That similar meaning is also conveyed between Aldrich Killian and the real meaning behind The Mandarin. In other words, Killian was Tony, while The Mandarin was just Killian's suit of armor.
"I am the Mandarin" was what Killian said to Tony during their final
, which did two things: 1st it highlighted probably Mandarin's most famous line "You'll never see me coming." 2nd it also reverberated the last thing Tony said before the end.
And despite that Tony no longer has any Iron Man suits to wear, it didn't matter anymore because like Tony said in the end of the film even without the armor, "I... AM... Iron Man".
On a different sidenote, Killian was the human version of The Mandarin's Fing Fang Foom complete with dragon tattoos and fire breath!