Since this thread has been revived, I would also like to talk about his famous OG Magic and how (IMO) ruined the entire plot of the series. For me, the main problem with Mel's OG magic isn't the fact that exists. As a concept, I find it quite believable that he had one since the start that wasn't Full Counter. Way before his OG Magic was revealed we already knew that each top class demon had a magic that couldn't be shared or used by anyone else in the clan (for example, Derieri's Combo Star, Galan's Critical Over, etc). That is an original magic. However at the moment we learnt that Chandler could use the same technique as him and the fact that it was him who taught to Mel that spell in the first place everything became pretty suspicious and gave room to think that FC maybe wasn't Mel's true magic. Back to the topic, the main problem Mel's OG Magic has is the reason given to explain why he didn't use it until now.
And yeah, the reason he gave to us is that he was afraid. Ok, cool. Afraid of what exactly? You cannot give such vague answer to a concept that could potentially destroy the main plot of the series. Most likely it had something to do with not being able to see Eli again, since he couldn't be on the mortal plane anymore after activating it (at least, that's what is implied). But why would he be unable to see her again? It's not like she couldn't come to the Demon World with him. He was about to leave with her in a frail human body before Boulder-Kun attacked. Alternatives existed.
Meliodas being afraid of using his OG Magic implies that it's something he could have used at any moment (which btw, it also makes the "I need the 10C to be on the DK's level" plot point completely nonsensical). So we have that Mel had all the time a power that was beyond the DK and the SD. With that power it isn't far-fetched to think that he could have ended the Gods' whole career during the Holy War if he truly felt like it (at least he had a decent shot) and become King of the demons himself. If he became King (and maybe Eli queen since he could probably have killed the SD too) the conflict between those two clans would most likely have ended, or at least would have greatly diminished. Of course, I'm not saying accomplishing such thing would have been easy to do at all, but the possibility was there.
All of this leads us to: no curse plot (Eli wouldn't have had to die more than a hundred times and both of them wouldn't have had to go through all that pain and suffering), no massive brainwash for OG Gowther's part, no Coffin of Eternal Darkness and most likely some coexistence between the two clans. This would also mainly solve the problems caused 3000 years later like the anihilation of the Fairy King's forest by the enraged demons, the Commandments attacking and sucking the soul of every citizen of Britannia and a lot of other plot points that were caused by the outcome of the Holy War.
The fact that the majority of the events and plot points that have happened could have been avoided (or at least partially negated to some extent) if Meliodas tried harder is in itself ridiculous. I used to really like Mel (he was probably my favourite character of the series at some point of time) and I think he had a lot of potential, but the way Nakaba handled his character and his power in these lasts arcs (mostly in the Second Holy War arc) is just absolutely horrid in my eyes, to say the least. To add more salt to the injury, Nakaba went and took that same OG Magic that ruined the plot away, without any explanation whatsoever about what it was, what uses could have had in combat, no backstory or insight as to why he was so afraid of using it to the point that he rather prefered his dearest person to die over a hundred times. Nothing. Absolutely nothing. The concept disappeared as fast as it appeared.
And you know what's the worst? Nakaba could have avoided all of this by simply changing the "I could have used my OG Magic all the time but I was afraid of something" to "I wasn't able to use this magic because I wasn't strong enough and the requeriments for the training to reach the DK's level and thus unlocking it, were finally met in Purgatory". If Nakaba had explained it in this simple manner, for me it would have been more than enough, but I guess this wasn't good for him. Basically, in my eyes Nakaba sacrificed the main plot that he created and molted for hundreds of chapters for Mel wank. A wank that basically destroys any kind of tension, drama and consistency and makes this manga that I once fell in love with a complete mess. Of course Mel doesn't have all the fault (some of the other Sins and factors also share some), but I think that he is, by far, the biggest player in this scenario.
PD: Sorry for the wall of text, I got a little bit too carried away