I'll start by saying I generally enjoyed it. There's a lot I disliked though, which I'll list a few points.

Firstly, the main character is a psychopath. He shows virtually zero empathy for his friends.

No character development means if you didn't like him at the start, you won't like him at the end of the first season. It makes understanding why Mikasa cares for him confusing, even with exposition shoved in your face. Not saying he has to be a Mr. Nice Guy, but throw us a bone here!

Secondly, the first season is about 10 episodes too long. There's too much filler and pointless side stories. Then the final plot twist is just way too obvious.

I could rant and nitpick for days, but in the end, the show is okay. It's just not quite there.