Moral ambiguity, a corrupt galactic Federation, terrorists-as-heroes, story and character arcs. Unfortunately, a lot of those episodes were visually comparable to the Doctor Who episodes of the time. Plus, some episodes were stinkers, too. If someone new to the show caught the wrong episode, he’d probably swear it off for life.
Luckily, the folks at io9 have put together How To Get Into Rebel Space Opera Blake's 7