Meliodas' sword's hilt is shaped like a dragon with its throat being the place where one normally holds it. In eastern mythology, dragons have a "reverse scale" on their throats and anyone else touching it sends them into an Unstoppable Rage. Considering these draconic motifs and Meliodas' title of Dragon's Sin of Wrath, this makes sense in a meta way.
At one point, Meliodas tricks Elizabeth into thinking that the next location they had to attend, she would have to join a Naked Dance Festival. Several chapters later, Elizabeth ends up shrinking, unable to fit into her clothes, and spends most of the time naked with only a ribbon to cover her.
King never used his real appearance and name during his time with the Sins because he wanted to prevent Diane remembering him from their time together when she was a child.
Diane and Meliodas recognizing King in his real form makes sense despite the above as King was Diane's Childhood Friend and she could have recognized him subconsciously, and Meliodas was one of the soldiers who arrested him 200 years ago.
The two times wherein Ban was imprisoned were both after a person important to him disappeared. The first time was after Elaine died and the second was after Meliodas disappeared and believed to be dead. It would seem that Ban willingly let himself be captured in order to let the pain out.
After Byzel, Elizabeth became sick and was sent to bed to rest, however it would seem that Elizabeth was in love with Meliodas as she blush harder when Meliodas is close to her. This would explain why Elizabeth suddenly feels better.
On the cover of Chapter 19, Elizabeth and Diane are shown in each other's clothes. This foreshadows what happens in Byzel wherein Diane had to wear Elizabeth's clothes as result of them being both shrunk, and unable to fit into their old clothes.
When Gowther first meet Dreyfus in Liones, Gowther perform a pose while giving his name and title. This is the same thing that Pelio asked Gowther to do back in Ordan while they were playing which shows that Gowther is honoring his time with Pelio.
It makes sense that Guila isn't transforming into a demon as quickly as Jericho or the other 2nd generation Holy Knights. Guila lacks the Inferiority Superiority Complex that drives the other new generation Holy Knights — the only reason she drank demon blood was because she was willing to do anything to protect her brother.
Strong negative emotions are needed for the New Generation Holy Knights to fully awaken their powers. For Guila, those were the feelings of abandonment brought about by the disappearance of her father. By then she had an inkling that "Doting Dan" did not abandon them willingly, resolving much of her issues. This probably contributed to her resistance to the Demon Blood transformation.
One of the reasons Ban got along with Meliodas is because he reminded him of Elaine. They're both youthful, blonde and treat Ban well despite his reputation.
Diane acting like a teenager (despite being one of the oldest Sins) makes more sense when you keep in mind that giants age really slowly.
Regarding Drefus being possessed by one of the Demon Clan. When Gowther used Nightmare Terror on Drefus he is approched behind from a unseen shadowy figure who surprised even Gowther the figure was likely the demon possessing Drefus who stepped in to prevent his host from being rendered useless and wouldn't of been caught up in Gowther's illusion as it was targeting Drefus alone.
Helbram says he uses his human form to remind himself why he hates humans. But, if you think about it, it's also the worst self punishment. Imagine how he must feel looking at the face of his friends' torturer and murderer whenever he looks at himself in a mirror.
Vivian is obsessed with Guilthunder and could force him to do anything. Do the math.
Guilthunder knew all along who his father's murderers were and had to see them everyday in the work and pretend normality. Now, Hendricksen is only his superior but Dreyfus, beside that, is his uncle. Try imagine them having normal uncle-nephew talks and seeing each other in family reunions, especially if Zaratras's topic was mentioned. With Guilthunder knowing Dreyfus was a murderer,unable to speak and perhaps even fearing Dreyfus could kill him or Margaret if he discovered they were in the secret. During ten freaking years.