Whoa. So. Ummm. Huh. Whoa. Man. The questions, they are a-swirling.
Reeeeally interesting episode. All philosophical and what-not. I'm not even gonna begin to go into that part. But, there are a few myth-arc advances, and now I have many theories.
First off, my slightly-joking, tongue-in-cheek theory: So, the person that Dean's deal brought back DEFINITELY isn't 100% Sammy. No, it's Sam with an extra slab of cute-puppy-dog-ness. I mean, last week was just for teh funneh, or so we thought. But it kept up this week. Just kinda wanted to pinch his cheeks, or like, scratch him behind the ears or something. Awwww. However -- perhaps he's simply saving up the evil. Extra-cutesy most of the time, but then when he goes dark, he reeeeeeally goes DARK.
For my slightly-more-serious theory: Demon!girl has some unknown agenda, right? An agenda much larger than getting her name actually said out loud on the show. She wants power, I assume. Now, the demons are all running chaotically without a commander, as Sam never stepped up. So, I think that "demon!girl"...oh screw it, Ruby... is manipulating him, big time. She's feeding him information, she's getting in close. She's just given him an immensely powerful weapon that he's already firing off willy-nilly. If she gets in close to him and can push him to the dark side, bit by bit... then suddenly she's second-in-command to the demon armies, with Sam as their general/king/leader. Even Dean's already beginning to question Sam. Ooh boy.
Also, the YED has a name.
Azazel. WHOA NELLY. "According to the Book of Enoch, Azazel was the leader of the Fallen Angels that bred with women before the flood, fathering the race of giants."
This show just got a hell of a lot awesomer.
ETA: Oh, hahah... so, I've been reading some other theories. Apparently some people think Ruby IS Lucifer. Could be, could be. Seems a little strong to me, but who knows. Meh, if too many people catch on, Kripke will just change it anyways. He's TOTALLY lurking in fandom.