Which is odd considering Vibranium (the material cap's shield is made of) is supposed to be unbreakable practically.
Hokay:
1) When Fury is debriefing Stark about the Avengers Initiative, there are blips on the eastern coast referencing the Hulk, in the north in a polar ice cap referencing Cap, one in the sea referencing Namor the Submariner, and one in the middle of Africa referencing Black Panther. There is also one in Europe possibly referencing Red Skull.
2) The man who gives Ivan Vanko (Whiplash) his fake passport says he is a part of the ten rings, a reference to the Mandarin, another Iron Man Villain.
3) Ivan Vanko's passport says he is Boris Turgenev, who was the second Crimson Dynamo (yet another Iron Man Villain). In the comics, Boris came to America with Black Widow in order to kill Anton Vanko.
4) A scene from The Incredible Hulk is playing on one of Stark's monitors in his office, Nick Fury also says he has "a bigger problem."
5) The briefcase is a reference to the fact that Tony Stark used to keep the ironman armor in a briefcase for quick use, and the fact that it was silver is a reference to the Silver Centurion Armor Tony used in the comics to defeat the Iron Monger (the villain from the first movie: Obadiah Stane).
6) The man who speaks to Tony about an idea for an electric jet says his name is Henry Pym, another future avenger.
7) Obviously a reference to Stark's alcoholism with the constant blood level checking, and then the eventual drunken scene pulled A LOT from the Iron Man comic "Devil in a Bottle." Rhodey in the comic also stops Tony from endangering civilians and pilots the iron man suit while Tony is recovering.
I didn't catch this one, but apparently during the race the second driver from the UK is named Chapman, a reference to the character Union Jack.
9) Shield asks Tony to become a "consultant" much like he becomes during the crossover event Marvel Civil War. Tony eventually becomes leader of SHIELD
10) Thor's hammer is found in New Mexico because that's where he came back in his most recent run due to his alter ego Donald Blake finding the hammer there.
11) Not really a reference, but whoever designed Tony's suit took a lot of reference to Tony's (almost) latest suit Extremis

Also: The person DJing at Tony's Party was the actual DJ AM, who is now dead. He played a song I believe off the DJ Hero soundtrack which he helped create.
Furthermore in the comics:
1) Tony Stark now has a repulsor rig in his chest, much like Stark in the movie.
2) Pepper is a love interest again.
3) Pepper is the CEO of Tony's newest company.
Getting closer and closer!