[hider=POSSIBLE SPOILERS]From what I can remember, a certain Titan moving much, much faster than everyone else and not being seen to make it back to a location was one of the main ones. Oh, and humans never winning a single battle against Titans, despite having several soldiers clearly capable of solo'ing several at once.

And the anime started with a large population of humanity's remaining numbers getting slaughtered, and it ended with them no closer to winning than they were at the start. Every conflict between those points pretty much ended with the Titans killing yet another large percentage of humans, or screwing them around in a way that they can't learn anything new to fight back with. Plus they killed off the only characters I gave half a crap about.

The series basically spent 26 episodes saying "lol fuck progress".[/hider]