The main story line is pretty bad... until the chapter ~100. So you have to survive this because what comes next is just so epic! I'm looking forward to learn about reincarnation, Meliodas's past and Escanor. Yes, yes, the main feature of Nanatsu is that friendship/love surpasses death through reincarnation - watch this video (https://www.youtube.com/watch?v=qd4pMykGn9g). As I said before, the main story line is retarded in the beginning. But Ban's Side Story and King's Side Story are just super awesome. The worst things about Nanatsu are fan service and ecchi elements. Ecchi looks completely unsuitable. Many people complain that the main characters are overpowered. Yes, they are...till chapter 100.

Also, the anime skips a lot. That' why manga >> anime.

My scores:

Story: 5/10 (but it will be 10/10 later, I promise)
Art: 8/10
Humor: 8/10
Characters: 8/10 (Ban, King and Escanor are super awesome)

Overall: 8/10 (but it will be 10/10!!!)