Number 21 on the IMDB list of the top 250 movies? Whoa.
It's not awful, but it's utterly shameless in its 1950's liberal preachiness. The plot is simple: a jury trying to reach a verdict in a murder trial. The defendent is a slum-dwelling generic minority, accused of knifing his father. Henry Fonda plays the saintly Juror 8, wearing white, lost in thought while all the other jurors simply want to vote guilty and get on with their lives. Juror 3, his nemesis, is played by Lee J. Cobb as a sweaty nutjob who wants to find the accused guilty as a misdirected lash-out at his own estranged son. Every other character is equally zero-dimensional and usually stereotypical.
On the other hand, it really is a pretty gripping situation: one guy brave enough to face down eleven guys and gradually win 'em over by force of argument. Steven Seagal wouldn't make a good choice for Henry Fonda's role when and if they do a remake.
Trivia: of the actors playing the twelve jurors, only the Jacks (Klugman and Warden) are still alive.