Co-op is currently available in assassinations and missions—very short quests primarily designed to give you crafting materials—and will come to the story campaign "very soon".
Future updates will add four-player co-op, more synergies between heroes, more "co-op systems", special co-op rewards and drops as well as balance improvements, Mad Head said.
Both Tom and I enjoyed our time with Pagan Online, which is currently in Early Access. The combat is crunchy, the crafting satisfying, and you have plenty of skills to play with, as well as a stable of varied characters to unlock. The structure is reminiscent of Warframe: you have a choice of different mission types, including quests across its eight-act campaign. You're transported back to a main hub after each one so you can craft new gear.
As Tom mentioned, the skill upgrade system is basic, and it's annoying that new loot doesn't change your appearance—you can only do that via skins. But if you're a fan of Diablo and its ilk, it's definitely worth considering.
The Steam page is here.