I've just recently read about the spoilers for the season finale of Supernatural and somehow people seem to be shocked about someone important potentially dying. But really, why so surprised? If we look at the last three season finales (and the first episode following after) EVERY time someone had to die, and not just the baddies but the characters dearest to us!
Not convinced? Well let's look at it:
Season 1 finale: All the winchesters left for dead at the side of the road
2x01: Dean dies, but John makes a deal with YED and dies instead
Season 2 finale: Sam dies because Jake stabs him and Dean makes a deal, effectively killing him 1 year later
Season 3 finale: Dean dies because of the Deal
4x01: Castiel 'raises him from perdition'
Season 4: Someone IMPORTANT will die
But really, after they exhausted their share of Winchester sacrifices 'Someone' had to die that for once wasn't a main character.
Personally I'd prefer it if it wasn't Bobby, but as he's the one most like family I'm fearing the pattern, Ruby would be logical as her purpose as the one leading Sam towards his dark side has come to an end but it would also be the first time none of the 'good' crowd were killed in the finally, ah well, let's hope they break the pattern then.