Ability Over Appearance: Stanley was originally written as an older man, but Elia Kazan realized Brando would be perfect for the part and Tennessee Williams agreed, saying it gave the character more dimension for his violence to seem to come from youthful ignorance rather than aged spite.
Beam Me Up, Scotty!: One of Streetcar's most famous lines is "I don't want realism. I want magic!" Although it was added to later versions, Blanche never says this in the original play. Here's how the conversation actually goes:
Blanche: I don't want realism.
Mitch: Naw, I guess not.
Blanche: I'll tell you what I want. Magic!
Executive Meddling: The 1951 movie suffered heavily from this: besides changing the ending, they downplayed the plot point of Blanche's teacher/student relationship (though it's still a plot point as to why she's not working) and removed all references to Blanche's young love being gay; instead, he's called a poet and "sensitive", and the reason for his suicide is slightly changed from being found out by Blanche and calling him weak, to Blanche being an Alpha Bitch and openly disdaining him at a prom. As for the ending, it changes the tone to a slightly more hopeful, though very uncertain, one - Stella decides to leave Stanley, and it fades to black. Compare the play, where, despite being openly molested by Stanley (who thinks of her less as the mother of his baby and more of a sex toy), she stays with him.
The Red Stapler: T-shirts sales in general, and tight t-shirts especially, spiked as a result of Marlon Brando wearing several in the film and play. This is partly what caused tight t-shirts to actually be mass produced; until then, only regular t-shirts could be bought, and for the film regular t-shirts were washed several times and sewn in at the back to be tighter.