Risa

... And it turned out Tasha really died, though the article did mention the actress returning on a few occasions. It sounded like the first season was a little rough behind the scenes, and the actress was also worried that the role would be too repetitive(?)

Ultimately, I feel kind of bad for the actress since TNG is widely revered as one of the best sci-fi series and she missed an opportunity to be a major part of it; but I can understand why she felt the need to leave. Plus this change gave Worf more time to shine, and also made room to introduce Guinan; so I really can't be mad.

I also still find it pretty touching when the cast references Tasha and mourn her (like Data's hologram in the episode where they legally have to prove his sentience).