Perhaps the biggest issue that I find with it is that the cast consists largely of shallow stock characters. For me this is what really kills the anime, I think. I just couldn't find it in myself to care about anything that happened because of them. This wouldn't necessarily completely sink an anime on its own, though it would have to have some pretty significant strengths in other areas to make up for it, and Attack on Titan doesn't seem to have any. The story of a young man being given responsibility over the fate of mankind due to a plot contrivance giving him sole access to or rare talent in some kind of great power is a very common one in anime, and within the first season this show does very little to mix it up at all. The tone is pretty consistently bleak, with levity being rare and unfortunately rather poorly implemented when it does appear, but for an anime that's so lacking in depth or original thought this just serves to make it even more boring.
Its being so depthless and unoriginal is exactly why I find it difficult to understand why people are so interested in it. What about this show makes it apparently so much better than the mountain of other anime that have used the exact some concept and used it, to my mind, just as well? I suppose it could be the fight scenes, which are fine, though not particularly noteworthy if you ask me. Or perhaps the consistency of tone, which is honestly rare in anime and consequently kind of refreshing even if it doesn't really help the show much. But then I guess that the reason I'm writing this is because I don't know why this show is popular, so I certainly don't have any confidence in either of those guesses. If you enjoyed this show, don't hesitate to tell me what you thought was good about it. I'm honestly interested to know. For now though, I'm scoring it a five out of ten - I'm completely apathetic toward it. Because it does nothing at all unusual it can neither be said to be at all good or really at all bad. It's just mediocre, which when you get down to it is really the worst thing to be. I certainly don't intend to watch any future episodes of this anime.
