Tasha Yar

Sigh. Tasha Yar was cool. And friggin' attractive. But then she rebelled againt Landru's policy of "no cool stuff in the scripts". Faced with the prospect of living under Landru's thrall or death by copy toner, Tasha chose the copy toner. It was all a fairly crappy affair, and tragic.
You see, if Yar had waited another season or so actual script writers would have gotten around to her and given her the scenes and characterization she so richly deserved. She joins the ranks of Hoshi Sato and Torres as cool hot chicks on Star Trek ignored by the creators.
Because of her death, we got Troi shoved in our faces unerringly for six seasons and 4 movies.