It's simple, except of the thrilling factor SnK really offers nothing to the viewer!
Story: After reading the synopsis SnK should be a simple action anime, based on the revenge of Eren Jaeger. Sadly the Story makes so many ridiculous twists and turns that not a single unanswered question was answered instead more and more unanswered question came up. The best example is the last episode, I really don't want to spoiler so I will just drop a single word here: crystal. The people who have seen the end know what I mean.
Everyone knows that SnK isn't finished, you can read the mange and there will be a second season, still the first season story, was completly pathetic. So only based on the first season story, SnK had really the worst storyline I have ever seen in my life. Also included a lot of filler episodes in which the storyline just stopped. (this is a personal comment: it really breaks my heart to see people rating the story with 10/10. Compared to DeathNote/CodeGeass/Steins:Gate/... which really had well thought out stories (most of the time))
Art&Sound: The combination of Art&Sound was the reason why SnK was thrilling. The perfect prospects of a situation included a perfect set and matching music gave us all goosebumps. Art was okay, nothing really special compared to other animes of that season.
Character: This part is as same pathetic as the story: Jean is really the only character which had a normal personality and wasn't killed after 2 minutes. Every other character is an eccentric or cannon fodder for the titans. So only irrelevant characters die.
SnK shouldn't be rated as outstanding by ignoring the badly made storyline in the first season. The most important thing for me to rate the anime as poor was the moment at the last episode when I realized that after 25 episodes nothing relevant happened to the whole mainstory.
