Note: May contain spoilers regarding plot and characters.
To start off, the story is painfully typical of a Shonen. The first few arcs consist of gathering a group of main characters, battling minor villains before a showdown with the evil usurpers of a kingdom, plot threads that have been done far better in Saint Seiya's Sanctuary Arc or One Piece's Baroque Works Arc. After this, a new set of villains are revealed to be the true masterminds with far greater power, as it becomes yet another series ruined by bloated, rapid power escalation/power creep with the main characters suddenly getting dozens of times stronger to face the new threat. All this could be excusable as being average Shonen if not for how contrived and pointless the series' "plot twists" are, such as one character first appearing to be a scrawny midget in a helmet to being a huge knight to being a Bishonen fairy, going far beyond the realm of a decent twist and becoming pointless overall. We also have the Sins having been forced to retreat against the knights of the kingdom in the past when they still had their weapons and were stronger than at the start of the story, yet from the start up until the end of the first major arc, they're able to easily handle the elite of the kingdom's forces even after they've powered up tremendously through being infused with demon blood.
Then there's elements such as characters being affected by things such as amnesia all for the sake of the plot.
Speaking of characters, this is what the series is mainly praised for, despite not having much to differentiate it from most other Shonen. Whilst all the main cast members have colourful personalities and fleshed out backgrounds, none of them gain too much development for the first 100+ chapters and even when a select few do, characterisation in the series is ultimately bogged down by many acting in ways to be shaped by the plot rather than them shaping the plot. For instance, the amnesia I alluded to before causes all investment in one of the characters to be lost when it acts as a character rewrite. It's not as though their colourful personalities are unique to this series as each one feels more akin to a Shonen stereotype. Meliodas is the perverted hero seen in many Shonen of the 80s/90s, Elizabeth is the useless fangirl who accepts anything the protagonist does, Ban is your typical bloodlusted, battle hungry member of the main cast seen with characters such as Vegeta in Dragon Ball or Kenpachi in Bleach, etc.
Moreover, the villains are even worse. In the first half of the story, most of them seem to only be evil for the sake of evil. In the second half, they're instead shown to have all their actions be irrelevant due to being mind controlled by a bigger threat, making any attempt at good characterisation amongst it's villains to become a joke.
Art is decent enough. It's distinctive enough from the typical manga/anime conventions with unique aspects such as the way Suzuki draws noses and eyes for his characters, plus he's become far better at making his female characters look more appealing for male readers since his previous work Kongou Banchou, but beyond that nothing stands out in the art as being great.
As a piece of entertainment, the series works well if looking at it as dumb fun and is perhaps the best depiction of Arthurian Legend gimmicks within manga, though that's also quite a dismay when my interest in such is great enough that this seems like a disservice to such Legends. It at the very least never falls quite as low in quality as most other Shonen of the 21st Century such as Bleach, but never elevates itself above the standard either and gets considerably worse after the first major arc ends, meaning only the first 100 chapters are worth reading.
Overall, Nanatsu no Taizai isn't a series I found worth investing any more of my time in and one being completely overrated by those who like to ride on the bandwagon of modern battle Shonen mediocrities. If you want to view Nakaba Suzuki's better work, read Kongou Banchou or if looking for a similar but better action-adventure, read the first half of Dragon Ball.
Plot - 3/10
Art - 5/10
Character - 2/10
Enjoyment - 3/10
Overall ranking - 3/10
