I love Grimm because I can't believe it's real. It resembles a fantasy story written by a 12-year-old who isn't suited to be a fantasy writer, but doesn't know it yet. I've been there. When I was 12 I loved werewolves, but I was too lazy to learn about wolf physiology or come up with any rules for how transformations would work. So all my stories were about "werewolves" who didn't actually turn into wolves. Now I discover that adults can get paid to write a TV show about werewolves who don't turn into wolves.
The "plot" of Grimm is that a police detective founds out he's one of a long line of monster hunters, which included the Brothers Grimm. One tagline of the show is "The Tales Are Real," implying that the monsters are going to be villains from fairy tales. For about two episodes the show actually tried to stick to this, but pretty soon it surrendered and the crimes Nick solves can now correspond to fairy tales, random contemporary literature, or nothing at all. The correspondence is usually really forced, like a feral child who uses her hair as a weapon is supposed to be Rapunzel, and this originally drove me crazy but I decided to just try and forget that the episodes were supposed to be based on stories.
Now I'm free to enjoy the incredibly low-concept monsters. Here is the low concept: some people are part animal. As far as I can tell most of them don't actually transform into the animal, they're just sort of like it. Occasionally this gives them abilities or conditions that are actually supernatural, like when a spider woman has to kill and eat men to avoid aging rapidly, just like the episode of Fringe where the same thing happened. But most of the time we learn that snake people are good lawyers, mouse people are shy, and rat people have a special connection to rats. The only thing that makes the average Grimm episode a fantasy is that at some point Nick's magic powers allow him to see a vision of a person's face turning into a CGI animal. Then he goes home and reads a book that tells him what personality traits that animal/person has.
Now that I think about this it seems really messed up. "I just found out that the suspect is black! I'm going to go to the library and look up what kind of crimes black people commit!" But you know, wolf people and so on are not real, so it's all in good fun and I've managed to remain unaware of this implication until now.
I think what I find appealing about Grimm is that it seems so real. The magic is exactly as boring as magic would be in real life. The monsters seem something other than human when they actually have powers, but they usually don't, and the Amazing Revelation of magic is basically: "Hey, you know that guy who killed someone for a reason that already makes sense? Well, his worldview was influenced by the fact that he was part lion."
Maybe it's more like Aesop's Fables than fairy tales, but the monsters are never able to be metaphors for humans because they explicitly identify as non-human and have their own culture. Grimm ends up being the most childlike show possible as it comes close to implying no human evil is really the fault of humans. Most murderers, rapists, etc. are just animal/people pretending to be people.
And you can't Godwin your way out of this, because in a recent episode, Nick was inexplicably watching footage of Hitler when Hitler's face turned into a CGI wolf! It all becomes clear: Hitler didn't hate Jews, he just wanted to eat them. I don't think this radically silly message was intended, even though when you think about it it's the only way to read the show. Its complete unawareness of how offensive and ridiculous it is makes Grimm the most adorable show on television.
Three-second review: I've been watching Lost Girl, which could be VERY lazily described as a combination of Angel and Buffy, and maybe Neil Gaiman. Not life-changing but it has been completely satisfying from the beginning. The worldbuilding is pretty good, ~strong female characters, gay and straight love interests treated equally, silly jokes, basically a good time.