Lots of familiar faces here--Lance Reddick is well-known to Lost fans as the mysterious Matthew Abaddon, but fans of The Wire and Oz also know him for playing key characters on those shows. Speaking of Oz, Kirk Acevedo also played a main character in that HBO prison series.
Mark Valley played Atty. Brad Chase in Boston Legal, and before that he was also on The 4400. Blair Brown had her own TV series in the late 80's, The Days and Nights of Molly Dodd.
As for the two male leads, Australian actor John Noble, who plays Dr. Walter Bishop, had his most famous role to date as Denethor in The Lord of the Rings: The Return of the King, while Joshua Jackson played Pacey Witter, one of the main characters in the teen drama Dawson's Creek.