(Ultra compressed version at the end of the review).

Nanatsu no Taizai's greatest strength is, I think, in the way it sincerely expresses emotion. It doesn't matter how messed up a character arc, the power levels or the greater narrative are - a look, a few words, a short backstory are all it really needs to make you resonate with the character at a deeper level than before.

Actually, for all its structural and narrative weaknesses, to me this manga makes more sense than most mainstream shounen. It's just that the author himself held it back by insisting on making everything seem like the biggest deal ever and you only realize it wasn't nonsensical shounen crap by the later arcs, thus everything looks way dumber and more inconsistent than it would have been if the author had taken any writing classes.

Had it possessed a more proper structure on top of its natural ability at creating hype and sentiment it might have been able to stand among some of the better battle manga - and even like this there's plenty of things in which it puts other mainstream series to shame.

Surprisingly enough, what makes Nanatsu no Taizai shine the most in comparison to its peers is the notorious "power of friendship" which is usually seen as a plague.
Most of it has to do with how natural the character interactions are. The Sins feel similar to groups such as the Straw Hats from One Piece, but with the advantage of their friendship being on the forefront: being a character-centric story, the progression of its protagonist is central rather than being diluted due to the overwhelming presence of a major all-important plot, which in 7ds is the one forced to take a backseat.

As someone who cares much more about characters and feelings and much less about the greater plot, and as someone who would've still been a huge fan of the series if it was just them journeying mindlessly, 7ds works wonderfully when it comes to sheer enjoyment - and the bonds between the main group feel especially heartwarming. Each of their character arcs puts an huge emphasis on emotion, but it rarely felt fabricated precisely because of how honest each of them was and because the author made it look like he cared about them instead of throwing them away in the later arcs to give more space to new, bigger things; even the final saga, the great big total war, was nothing more than a succession of character arcs, even if tainted by the derailing of the power levels.

One battle shounen which I feel tried to do something similar was, obviously, Fairy Tail. A lot of it has its heroes mostly looking like a bunch of shitposters, bringing chaos and adventuring and giving so much space to their "family" narrative, and I even felt like it worked for quite some time. I also feel like they ended up mostly targeting the same fanbase (and the same haters).
One thing that made FT fail for me, however, was how much they just beat you over the head with it. After hundreds of chapter with the same rycicled scenes and big friendship speeches it starts feeling fake, especially because of how absurdly possessive and unreasonable they were and because, even with all that, the cast was simply way too big and it was inevitable to throw away so many of them.
While Nanatsu had its fair share of "shounen moments", they rarely passed the line and managed to stay hype as fuck until the very end - with Escanor's big moment especially shining.

It's also worth noting how they usually don't gain random protection from evil or further boosts in strength because they want it so much, or because their friends believe in them even more than how they were believing in them before.
The only such case is King, who is a non factor because of how much thought went into him: first, it's established very early on that his strength is dependant on his emotions, and growing from immature forms to more mature forms in desperate situations is a characteristic of his whole species; second, there are only two cases in the whole manga where he solves an hopeless situation with such a power-up, making them way more meaningful; and finally, it complements incredibly well what his Sin is and how it affects his character arc. As the Sin of Sloth, King's whole deal is that he needs to become more proactive and stop holding back both himself and the others with his attitude; it goes without saying that he can only unleash his true potential when he puts all of himself into accomplishing something, and in his case the only trigger which can make him surpass his limits is not wanting to lose any more people because of his weakness.
This ties in with the overall point I wanted to make: the friendship and comradery between the Seven Sins isn't so powerful because it gives them the strength to fight tremendous enemies, but because it gives them the strength to face themselves and overcome their own weaknesses - which is why they're then able to stand against such enemies with renewed pride and courage. They're all "sinners", broken people who managed to find comfort in each other, and it didn't happen in a cheesy way nor did it require lengthy flashbacks to justify their friendship. They just met each other and started hanging out, like in some kind of safe space, having fun while never judging the others for their past, slowly forming their bonds. The "friends" in Seven Deadly Sins act as some kind of anchor, a comfort zone which gives them the support they need to overcome themselves.

The only reason this worked so well is because of how much the author himself seemed to care about their relationship. It's an old story in this genre: "friends" are the most important, the protagonists care so much about friends they would sacrifice everything for them, they're the most precious thing in the world... and yet it's rare to have all that much space dedicated to them just being themselves and acting as actual friends. This is mostly because battle shounen tend to favour advancing the plot of the arc, or reaching the fights as fast as possible; the slice-of-life scenes are easily forgotten and left behind as soon as the author really gets into telling his story, and all those so precious friends really only matter for dramatic moments or for their usual gags, at times not even seeming to care about each other by saying things like "it's him, I'm sure he's all right" or "let him do whatever the fuck he wants I don't care about him", no matter if they're facing God itself.

This approach is definitely what I prefer too see in such shounens, and combined with the sheer hype of the fights made me enjoy this manga way more than I should have.


TL;DR: Nanatsu no Taizai is a mess, but its strengths such as the natural way it shows emotion and friendship make it stand out compared to other battle manga which are more dedicated to the narrative.