The Flash has not been really explored on-screen since the character was created back in 1940, except for a 1990 live-action television series starring John Wesley Shipp. It was cancelled after 1 season, though, after poor reviews and low ratings. As there is no other strong in-person performance with which to compare Miller, he will only be competing against written words in comic books, a decidedly easier task.
It is, probably, not an obvious choice, to the point of being regarded by many as controversial, but I'm sure he'll do as fine job as any other man. He's a good actor, and his physique will give the movie an extra dimension: its manga-like angular fluidity subverts conventions of 'superhero' genre.
Right now the landscape of superhero movies seems to be in state of flux. There have been a slew of Marvel rumors pointing to Captain America and Iron Man facing off in Captain America 3; Avengers 3 changing the roster; and possibly big crossover event films like Infinity Gauntlet becoming part of the game plan. On the DC/Warner Bros. side, we’ve been hearing everything from character team-up films (Batman V Superman) to full team up films (Justice League) to traditional solo character stories (Batman and Green Lantern Reboots, Man of Steel 2, Aquaman).
Ezra Miller may be young, but he has already built quite a portfolio. He has had numerous roles in movies and television shows that have allowed him to grow as an actor. Portraying a homicidal character in "We need to talk about Kevin" and his supporting role in "Perks of being a wallflower" have showcased his diversity as an actor. Ezra Miller is sure to bring The Flash to life.