Following the Rob Morrow theme:
David Krumholtz: Lost his gig as Morrow’s Brainiac brother, Charley and hasn’t done much of note since.
Self Induced exile due to swelled ego:
Sasha Alexander: Thought NCIS was all about her. Left after three seasons. Is just starting to come back as Dr. Maura Isles in TNT’s ‘Rizzoli & Isles’.
Genevieve Bujold: Was originally cast as Captain Janeway in ‘Star Trek: Voyager’, but couldn’t handle the twelve to 16 hour shooting days. The role went to Kate Mulgrew.
David Krumholtz: Lost his gig as Morrow’s Brainiac brother, Charley and hasn’t done much of note since.
I read that she couldn't stand the very long hours and thought the cast was overworked. Mark Harmon complained about the situation, too. Apparently the producer, Donald Bellisario, is a perfectionist who demands perfection from his actors and everybody else. In order to keep Harmon on the show, Bellisario finally agreed to turn the day-to-day reins over to somebody else.
By the way - trivia note: Donald Bellisario served in the Marines with Lee Harvey Oswald.