I really think that attack on titan has a great plot and a amazing world building. However, the reason that stopped me from give it a 10 is the complete lack of character building until the most recent chapters. Eren, the main character, takes 90 chapters until he finally do something to move the plot by his own decisions and is more like a plot device than the main character. Mikasa only say Eren and fight to protect him all the time, and other characters are either defined as psycophats, cowards or people that are decided to do anything for what they believe. In fact, i could say that 90% of the cast are more plot device than characters.
However, despite that, the mistery is really interesting and the fights are well done.