Marvel pays the rest of the Avengers cast a fraction of what they pay him. What does Robert do in response? Knowing that they are not willing to risk replacing him, he threatened not to return for any more Marvel movies unless they paid the other actors fairly. Also a Crowning Moment of Heartwarming.
Also counts as a moment for Joss Whedon, who also said that there wouldn't be an Avengers 2 if Downey couldn't be Iron Man.
Way back when, after watching The Matrix with his (producer) wife, RDJ stated, "I can do that." The rest is history.
Managing to make Blackface work in Tropic Thunder.
As of 2014, he is the top-billed actor in the two films with the highest opening-weekend totals of ALL TIME: The Avengers ($207 million) and Iron Man 3 ($175 million). The man opens big.