I did some story coop with my son yesterday and today. You start off on Bronze and there are 10 missions. Each mission has 3 levels. You can unlock Silver and Gold which increase enemy damage and limit/take away the ability to restart if everyone dies. And, of course, gives you better loot. It's different than the single player game in that you only have 2 ghost weapon slots and only one stance to start with. You get resolve but not for healing. There are healing drums random located on the level you need to use to heal (Ronin is like a portable healing drum too). Resolve is for unlocking the special ability of the class you play. You also get a unique skill with each class that can be used after cooldown cycle. The classes are Samurai, Ronin, Assassin, or Hunter. All classes still have classic swordplay and bow/arrow capabilities. They mainly differ in just special skills and abilities. The missions are varied and interesting. There are a lot more magical elements than in the game. So you need to do more than just slash your way through enemies. I haven't done the challenge levels yet. But my son says they are more like capture and hold areas of the map while waves try to push you back.
There is a lot of variety to gear and ghost weapons you loot. Many of the ghost weapons you can get don't even exist in the single player game. I haven't even played much yet so there is a lot to discover. But it seems like a pretty simplified online RPG game. Which I like. There isn't massive amounts of inventory and skills to manage. Each class does have it's own unique skill tree. But it's small like in the main game. I've only played Bronze level and did die a couple of times. But, it is pretty easy to jump in with no gear and get going fast.