Typing my own thoughts before reading all yours...
I have to admit to mixed feelings here. Obviously parts of it were magnificent. But I don't feel like the plot was "big enough" for the character issues it had to support. I'm assuming again that this was a casualty of the strike, and that if they'd had the full run, they could have built up to a much bigger and more dramatic climax than just running around somebody's house.
But then, maybe that's kind of the point - that it's one of the ordinary, everyday adventures that finally does them in.
I have to admit, I did NOT expect them to actually go through with it, that Dean would actually go to hell. I expected them to find some way out of it at the last minute, perhaps through the use of Sam's powers, but of course with some big price to pay, which would set us up for next season. And I like that they actually talked about both those possibilities, and then didn't do them. In retrospect, Dean actually recognizing the pattern I was expecting them to fall into is a master stroke.
I also was not expecting a cliff-hanger - I was expecting them to tie it all up in this episode. Which is my problem, of course, not theirs. Perhaps again it wasn't meant to be a cliff-hanger in the original plan, but they took the opportunity the strike gave them to make it one?
When did Lilith possess Ruby? The boys are surprised that Ruby got out of their Devil's Trap, and she replies, "There's so much you don't know about me." Now that was probably just a throwaway line to get her where she needed to be, but within-show, perhaps she was already possessed by Lilith at that point, who being a stronger demon, could break out of it? But then Dean sees her "demon face" when she first pops up, and that's what helps him to realise it's not Ruby later in the episode, which suggests Lilith hadn't taken her over by that point. Which leaves us with the question of how Ruby got out of the Devil's Trap. Did she use her witchy powers (separate from her demon powers) as alluded to in "Malleus Maleficarum" ?
On a related point, how did the father know Lilith wasn't inside his daughter anymore? Especially if the mother still thought she was? Or was he just meaning, "That's not my daughter" ?
Is Ruby gone? Or now that Lilith has vacated her body, can she just get back in it again?
Also interesting how Dean kind of flip-flopped on the self-sacrifice. He planned to fight it, to try to find a way out. When none of their plans worked, and it struck midnight, he was prepared to face it, calm and resigned and accepting. But then when he actually saw the hell hound, his simple survival instinct overtook all his noble intentions and he was desperately trying to run again. That last, instinctive feeling of fight-or-flight, of refusing to give in.