A lot of Supernatural's Sam fans get a little frustrated if he's not on screen or if he doesn't get to share a lot of scenes with Dean. While this has been true this season thanks to the arrival of the angels, it doesn't mean Sam doesn't have a storyline. He has a big one.
This season, while the angels have been making plans for Dean, Sam has been busy making his own destiny. In the latest episode, On the Head of a Pin, we discovered that Sam's demonic, psychic abilities have grown to an unimaginable strength. But at what cost?
Now we know his secret: demon blood. He was fed a few drops as a baby, but now he's chugging it straight from the tap to become so strong that he can outright kill a powerful demon with a single flip of the wrist.
While fans might complain that we haven't seen much of this transformation, I think that's exactly what the writers wanted. They've been hiding Sam's development into a powerful force the same way Sam has been hiding it from Dean. He recognizes that this is what he needs to become in order to stop what's coming, but he's ashamed of it.
Now that it's coming out and Castiel has seen first hand how strong Sam is, the sky's the limit. His story arc since Dean's descent into Hell has been profound, and just because we didn't get to see every step along the way doesn't mean we don't know what happened.
Sex and Violence put the notion of a Sam vs. Dean battle into the minds of fans, and with this latest revelation, that becomes an even stronger possibility. The Winchesters are headed down different paths, and while the two may argue along the way, the interesting thing is that the destination, at least in theory, is the same.
Sam wants to get stronger so that he can stop Lilith from breaking the 66 Seals. It's the same thing the angels want Dean to do, but he's having doubts about it. The biggest moral question on Supernatural this season is: do the ends justify the means? Is it OK to kill a human if it saves humanity? Is it OK to drink demon blood if it allows you to kill other demons? Would you sacrifice everything, perhaps even your own humanity, to save the ones you love?
For Sam, the answers to those questions is Yes. However, he'd be a lot happier if no one knew about it, which is why it's been kept a secret until now.