I loved Extant - I thought it was brilliant, imaginative, well-realised, well-acted, and if it doesn't at least get nominated for a Hugo next year then there's no justice in the world.
But...it told, as far as I could tell, a self-contained story, which played out over its thirteen episodes, came to a dramatically satisfying conclusion (and killed off several of the main characters doing so), and didn't really seem to be crying out for a second season (although the very closing scene kind of had "sequel bait" written all over it. But then so did the closing scene of Crisis...).
(Also? Whenever I see creator Mickey Fisher's name, I instantly think of comedian and Mock the Week regular Micky Flanagan. Which is wrong of me, I know...)