Warning: Spoiler Alert
I do appreciate the use of Ragnarök in this episode (especially with it happening on February 22, 2014 ... oh, I have a feeling that I'm going to have a great time on that day), but two things:
1) It's Fenrir ... not Fenris. Sure, Encyclopedia Mythica does show "Fenris" as an alternate spelling, but I have never seen that spelling actually used (whether that's in any of the dorky places that I've gone to do a little bit of academic research, or when actually reading things that have been written by people that currently follow any of the variations of Germanic Neo-Paganism, and yes, there are different "denominations").
2) Where the hell is Mjölnir? We know that the boys were in position of it at one point, and that Sam even used it (even though that would have been completely impossible for him to do without the gloves and belt that are necessary for even Thor to use it). So, where is it? I suppose that we can headcanon that it's inside the bunker somewhere (warded so that angels, demons, and Norse gods can't try to snatch it. And I would have loved to've seen them drive the Impala with it in the trunk, having the tail end of the car dragging as they went, but what with Show's redconning folklore and mythology in such a way that doesn't always look like they did their research, I don't think that would have happened), but for all we know, they just left it where they found it ... which would have been a stupid, rookie move on their part.
But both of those things being said, I do like the idea of Fenrir being a god to werewolves/lycanthropes. Who else would werewolves worship but a wolf? And it makes it even better that he's one of Loki's kids. Never mind that he's not actually a god, but just a super wolf that's the son of two Frost Giants; but the fact that he's supposed to take out Odin (amongst others) does make him rather formidable.
I'm not really getting the change in Dean in this episode. He's kind of gone back to the character that he was in the first couple of seasons (the shoot first guy). But after the whole thing with Benny, he really seemed to have changed, and realized that not all the monsters are really bad. Sure, he thought that something hinky was going on (and the fact that Garth is so gullible probably didn't help), but he went in wanting to kill things before he realized that there as anything going on. Maybe he was feeling protective of Garth? But that wasn't really established. Or maybe he's still kind of reeling from what's happened between him and Sam ... that might be the more likely.
Having Sam reference Ichabod Crane when trying to find Garth, I'm not sure that that would have happened if "Sleepy Hollow" hadn't become so popular (and since the fanbase of that show is probably largely made up of people who also watch SPN ...). But then, the writers on Show have consistently shown that they are at least partially aware of the crazy things that the fanbase is up to, so that was probably a bit of fanservice on their part.
I am glad that Sam saw the Mark of Cain and that Dean was forced to come clean with what had happened. I'm so very tired of there being lies between them all the time, and it would be nice if they were honest with each other for a while. But speaking of the Mark of Cain ... I wonder just how long it's going to take Crowley to find the First Blade. I don't want it to be too quickly, since that would seem unrealistic (which, I realize that this isn't a realistic show, but still), but I also don't want it to be right before they need it in the finale or something. That would just be dumb.