If you're the type of player who likes SP games, can devote a fair amount of time to the game and don't mind doing so, single player can be fun.
There's plenty to do.
You're going to want to enjoy building, farming (like crops), as well as mining and chopping and hunting and sailing and fishing, etc.
Co-op makes progression much faster. More fun? well that depends more on the people than the game I'd say.

NPC's are limited...unless you count Greylings. There's plenty of those that like to tap you on the shoulder and ask for a light when you're chopping trees.