A pretty interesting Norse based cooperative game that recently came out was Yggdrasil.
In Yggdrasil, each of the players takes on the role of one of the Norse gods attempting to prevent Ragnarok by keeping their enemies out of Odin's house. Each turn, the active player starts by flipping the top Enemy card - at which time the enemy represented on the card advances towards Odin's house, and also has a special evil power occur (such as bringing forward one of the other enemies, causing a frost giant to appear, or causing one of the Valkyries' islands to submerge). After this, the player is allowed to perform 3 different actions of the 9 available (one of the gods can actually perform the same action twice), and often the actions will culminate in an attack on one of the enemies (this helps knock them back away from Odin's house. The game continues like this until a certain number of enemies advance too far (one is in Odin's house at the end of the turn, 3 are across a certain barrier, or 5 are across a different barrier) in which case the players lose and Ragnarok ensues, or until the "Enemy" deck runs out of cards (at which point the gods have prevented Ragnarok and won the game).
The first thing that I liked about Yggdrasil was the number of available actions. Many other incredibly fun and challenging games (ahem, Pandemic) sometimes feel a bit repetitive because there are only a few actions that a player can perform. Yggdrasil has more actions than I have seen available in any other cooperative game, which really helps the game to feel like you have more options.
The next thing that I like about Yggdrasil is how the "vikings" work. When fighting against the enemies, a player must achieve a certain strength (normally 5) in order to win the battle. To do this, he can contribute any number of vikings that he has available (before rolling the die), add the value of his enemy-specific weapon, roll a die (which can add 0-3 strength), and finally add elves if his die roll was not high enough. This is all fine. However, what I really like about the vikings is how you get the vikings. There are 4 different islands that the Valkyries can go to. Each of these islands is represented by a bag full of vikings and fire giants. One of the actions that a player can perform is to move the Valkyries by one island (the closest islands have the worst odds of getting vikings), and then to pull out 3 pieces from the bag; any vikings pulled are kept by that player and available for future fights, but any fire giants pulled are put back into the bag (thus, getting a "good draw" stacks the odds against getting further good draws). There are also two other actions that a player can perform on his turn related to the vikings. He can take an action to move 5 vikings from the "deceased" pile into one of the bags, or he can pull 4 pieces out of a bag and remove any fire giants pulled (but put the vikings back). Therefore, the players are constantly adjusting the odds of getting the crucial vikings from any given bag - and if he neglects any of these actions, he could easily find himself unable to get enough vikings at a very critical juncture in the game.
The next definite pro of Yggdrasil is the number of players that it allows. Often cooperative games struggle with only supporting a very limited number of players (the new Lord Of The Rings: The Card Game only supports 1-2). Yggdrasil supports 1-6! What's more, I think that Yggdrasil would actually be fun with each of those numbers of players. Because of how weapons work (they are specific to each enemy, and you can only get the +2 weapon if you already had the +1 weapon for the corresponding enemy) I think that the different numbers of players would even add different strategic choices and conundrums.
The final pro that I will mention about Yggdrasil is how frost giants work. When a Loki card is flipped, he brings out a frost giant. The frost giants handicap the gods in some way while they are out (prevent them from performing certain actions or causing an enemy to become stronger). This in and of itself makes an interesting dilemma in the game, as players are forced to expend resources fighting the frost giants instead of the enemies. However, the frost giants can also be put together to form certain runes, which give the gods major bonuses (such as gaining 15 vikings that they can split amongst themselves). This causes the frost giants not to only be a pest, but also a potential source of "giant" boosts for the gods (hehe... I like puns) - players might even choose to fight the giants before they are even revealed by Loki in the hopes of completing one of these runes!
Now that the pros have been covered, I really only found a couple of complaints about Yggdrasil. First, this game is incredibly anti-climactic. Throughout the game you are constantly fighting off the enemies and frantically trying to strengthen yourself for the next fight. It can feel very exciting and can keep you nervous throughout the entire game. But the game ends by the enemy deck running out. You don't really accomplish a major goal like finding the cure for the diseases in the world, escaping an island, or completing your quest. You simply survive. You flip over the last enemy card, advance his figure, and then see that you didn't die - which means you lived. I realize that this is simply how it will have to be in this game because of how the mechanics work, but it still seemed a bit disappointing. I honestly have no suggestions for how I would change it, though, because I think any real change to this would drastically change the game.
One other thing that I should mention is a common difficulty in cooperative games. Since everything is fully known in the game, an incredibly outspoken (bossy) player can easily dominate the game and reduce the enjoyment of all of the other players. This really happens in a lot of cooperative games where "discussion of best potential moves" can quickly turn into "do it this way, or you're stupid." My fix for this - tell those people to shutup... but then again, maybe I'm the outspoken one in my group.
Overall, I give Yggdrasil an 8.5/10. I really enjoy this game. I think that anyone that enjoys cooperative games should definitely give this one a few plays, as I don't think that you'll be disappointed.
If Yggdrasil sounds interesting, you should check out Space Alert, Hanabi, and Lords of Waterdeep.
I would like to thank Z-Man games for providing a demo copy of Yggdrasil for me to try out.