Warning: Spoiler Alert
Timothy Omundson was amazing. AMAZING!! I completely loved him as Cain (and loved the inclusion of twisty Cain & Abel story). And I was kind of embarrassed by just how long it took me to remember what else I had seen him in before (as in Eli from "Xena," the psychic from Starship Trooper, and a really horrible Disney film that I'm not going to admit to watching), but when I went looking for his IMDB page, and I had a serious case of facepalm, and an "oh, yeah, duh."
I think that this may be the twisty of the religious stories that I may like the best ... mostly it may come from it not completely sucking (like so many other of the twistings that they've done), and just how well Timothy Omundson did with the character. I did rather enjoy the idea of Cain just being tired of everything and not wanting to get involved with everything much more than I like the idea of Metatron hiding out on Earth all this time. In a way, the two of them were doing kind of the same thing (in that they were hiding, cuz they didn't want to have people come looking for them, and they just wanted to be left alone), but the reasons behind what they were doing struck me as being very different (where one was tired of all of the killing and hate and the war, and the other was hiding out cuz he was too afraid to poke his head out).
I even like the twist despite the fact that the writers were very obviously putting in a parallel between the brothers (wherein Cain = Dean and Abel = Sam). I'm not a fan of one-to-one correlations or allegory, and typically this sort of thing would annoy me for the tiredness of what they keep trying to do with the brothers (since it's the same thing that they've been doing for years), but I'll admit again that a big part of the pass that they got on this episode had to do with the acting of the guest star, and the way that he was able to portray just how weary Cain was of the fighting. I probably shouldn't be so sympathetic toward a demon, but I am (after all, he was a complete woobie, and he needed to have a bit of a snuggle).
As far as Dean getting the Mark of Cain (and being able to use the First Blade to be able to kill Abaddon), I suspect that no good is going to come of that, and it strikes me as just one more way in which the Winchester boys are willing to throw themselves into Hell for no reason whatsoever. And the fact that now have it (cuz I'm sure that Crowley is going to be able to grab it out of the bottom of the ocean), it feels like we're getting just one more thing that is a super weapon (like Ruby's knife, the Colt, or the Angel Blades), and the fact that we're getting one more super weapon is getting really old (and feels a whole lot like they're cheating. Don't have a way to kill a super unkillable demon? Invent a new weapon that will do the job).
So, yeah, overall, the episode gets a pass cuz of the guest star, but not for the new weapon.