I learned to not get attached to any character not-named Mikasa, Eren, and Armin after seeing how fearless Hajime Isayama is at killing off characters. It's nice to see an anime / manga where the writer doesn't care if the character is a fan favorite or not when making those decisions. A lot of other series sometimes fall in that hole where the writer is afraid to kill characters off (big names like Naruto and One Piece are extremely guilty of this).
Then again, the titans are pretty crazy and dangerous, so a large number of deaths is expected even if it is mostly fodder.
And yeah, I was pretty surprised when I thought Eren died. Just seeing his arm get bit off to save Armin was gruesome enough.