Brian K. Vaughan, a writer of some of season four and five's more memorable episodes, has left the show. The writer, known for his work in the comic world (such as his series Y: The Last Man) was confirmed to have left the show "for greener pastures" by Damon Lindelof during a recent Q&A session.
Vaughan first joined the show in late season three, writing "Catch-22." The next season, however, he returned for many more appearances, writing "Confirmed Dead," "Meet Kevin Johnson," and fan favorite "The Shape of Things to Come." Most recently, Vaughan wrote three episodes of season five: "The Little Prince," "Namaste," and "Dead Is Dead."
I suspect that Vaughan left to pursue writing a film or television adaptation of his graphic novel, Y: The Last Man (a Spanish version of which Hurley carried with him aboard Flight 316). He has stated that he prefers to write his own creations like Y: The Last Man and Ex Machina because he doesn't think he's the best at using his "voice" with company-owned characters. Whatever the reason, Vaughan was a great member of the writing staff, and we wish him well in whatever he pursues.