((a few season 1 spoilers))
Story: 8/10 - It pulls you in with a zombie/godzilla type story that it teases with its first few episodes. It's intriguing, but not that original. However, past episode 5, this changes dramatically, and it takes a 180 by episode 8 and the direction changes almost entirely. See, this series pulls so many 180s that some are left disappointed because it's not what they originally expected, and this happens A LOT, especially in the story arcs past the first season. But let's not focus on past season one, though.
What started out as a story of the last bit of humanity fighting for survival quickly turns into flesh mechs, flashy reveals, betrayals, and WTF moments that aren't explained until later on. This series likes to show some bizarre, random ass scenes that either aren't explained until much later or not at all. It teases you with something about Eren's dad's basement, but leaves it too mysterious for you to care. It shows the Colossal and Armored titans at the beginning, but they don't show up for the rest of the season making you think they were a one-and-done deal. It raises more questions than it gives answers, leaving many frustrated; by the end of the season, when the main villain of the second half is defeated, she legit locks herself in a crystal, therefore giving NO answers about who she is or where she comes from. The villain is given hints of a backstory regarding her dad just as she's being beaten, but it's not elaborated on at all. See, the story teases you too much. It leaves you unsatisfied; the 4 year wait only made it worse. On top of all that, there's an end credits scene that acts as another 'reveal' but only leaves you confused as HELL.
Aside from those gripes, the story is good in that it is consistently interesting and well-thought out. It is very mystery driven at some points, and the build up to the fights is tense as well. There are pacing problems, though, so the episodes are best watched in a cluster.
Art: 10/10 - The animation of SnK is wonderful. It's among the best I've seen in any anime. It's movie-worthy. Studio WIT did a wonderful job. The manga's art is subpar, and even straight up bad for the first few volumes, but the anime just nails it. The backgrounds are always beautiful, the characters are always highly-detailed, and the fight scenes feel so smooth. Absolutely no complaints here.
Sound: 10/10 - Another perfect 10, Sawano did an amazing job on the soundtrack, and the sound effects are satisfying as well. Sad moments are occupied with tear-jerking OSTs, and intense moments are only made more intense with the soundtrack. It includes my favorite anime OST ever, Vogel im Kafig. Also, the ODM gear sound is very satisfying and it's something that you don't feel when reading the manga.
Character: 6/10 - I will admit right here and now, the manga, especially the later arcs, fix a lot of the problems I'm about to state. Doesn't excuse the fact that I hate how Eren, Mikasa, and Armin are characterized for the entire first season.
Eren is your typical shounen protag with a distinct goal, stubborn personality, and edgy "i swore to kill the all!" lines. Mikasa is your usual female lead that's obsessed with the main character - she is given pretty decent development in the first half (ex. ep. 6), but she's simply really annoying. Constantly being like "Eren! Eren! Are you okay?!" Armin is annoying, though he became my favorite character in the manga. He's your standard bookworm that gets bullied and such, and never believes in himself. He's obviously really smart and can think of good plans, but it takes half an episode for Eren & Mikasa to convince him to do anything. The side characters aren't given enough development until season 2, and the characters they DO focus on are pretty one-dimensional. This is the worst aspect of SnK season 1 by far.
Enjoyment: 9/10 - The first time I watched SnK, I was new to anime. I loved it. I binged it in just a few days. When I watch it with my friends, they love it as well. It's an anime a lot of non-anime fans watch. It's popular because it's simply fun to watch. The fights, glorious animation, soundtrack, and jaw-dropping reveals outshine most of the negative. Seeing the characters flying around on their spiderman-like gear fighting giant naked men is so intriguing and satisfying.
Overall: 8/10 - Very enjoyable anime with pacing and characterization problems. It's one of the most popular anime for a reason - the simple yet intriguing premise, and the quality soundtrack & animation. Don't go into it expecting a masterpiece and you should be pleasantly surprised. Again, a lot of the problems are fixed in later seasons/the manga, especially the characterization. I actually wrote an analysis of why I think Eren's characterization is genius.
