GameSpot UK recently had an interview with Pirates of the Burning Sea lead designer Kevin Maginn, in which they fired off a series of questions that looked to flesh out the game's concept for anyone not yet familiar with it. As well as name-dropping the guys from Talk Like a Pirate Day, some of Maginn's answers reveal a lot of the reasoning behind some of the important design choices that make the game what it is today.
One of these is the mainly solo-oriented mission structure in PotBS. In this case, it comes down to what Maginn himself preferred to have in an MMO -- he said that he enjoys group content, but wanted to have the ability to log on and achieve things on his own without needing to sink time into group setup. This brought about the system we see in PotBS now, with most missions aimed at a single player, but there are a few group missions that are much more elaborate than a regular quest and are repeatable, to satiate group cravings on a permanent basis.
Maginn also says that, apart from updates on a roughly monthly basis, a major content update should hit every few months. One aspect of the game not mentioned in the interview is the interesting economy and crafting system, surely one of the game's big selling points. Check out the rest of the interview at GameSpot.