User blog comment:God of nothing/Titan shifters-Invincible?/@comment-5047068-20131001194236

Titan shifters in human form are merely a lot more durable and hard to kill than normal humans, and also can endure worse injury, pain, and trauma. Bertholdt nearly had his jugular severed--might have, in fact--and Reiner lost an arm and had the other one split with the blade stuck inside it. Eren was able to survive with a wooden pylon through his chest and endure enough pain to focus and transform again.

However, had Mikasa actually cut off Bert's head or Eren been stabbed through the heart, they most certainly would have died.