How She Died: Killed on an away mission by an evil alien oil slick named Armus.
Why I Was So Sad: "Star Trek: The Next Generation" was my favorite show and Tasha Yar was my favorite character during its first season, because she was badass Star Fleet officer who proved that female officers didn't need to wear different uniforms to stand out (ahem, unlike Troi). Plus, I totally shipped her and Data, especially after the episode "The Naked Now," when she put on a sexy backless dress and seduced his android penis.
Just How Sad Was I? Well, I wrote a long letter to show creator Gene Roddenberry, begging him to figure out a way to woo actress Denise Crosby into returning (she did, eventually, as Tasha's daughter Sela and as Tasha in an alternate timeline). And then I wrote a letter to Crosby, telling her what she already probably knew, which was that she would never get a better role. But I said it nicer. I was 11.