I'm pretty new to co-op gaming, and have only really bothered with cooperative board games in the last year or so.
I suppose that roleplay (which I've been doing off and on for half my life!) is a co-op of sorts so I shouldn't be that surprised that I have been enjoying them greatly. Of course roleplay has a games/dungeon master who keeps the difficulty level balanced, ultimately letting the players win, the skill here is to make them feel as though they have won against great odds. Done well it lends an epic and heroic feel to the game, done poorly it can either undermine any sense of peril or just feel totally unfair.
But a boardgame doesn't have anyone to manage this and coming up with a game mechanic to emulate this is a bit hit and miss. People who play games want to win but they don't want it to be easy, there must be a sense of achievement. A poor co-op game mechanic can result in flip-flopping between ridiculously easy and brutally hard on the roll of a dice (figuratively and sometimes literally). How hard things should be is no doubt a matter of taste so the mechanic also needs some way to scale the difficult to suit the skill, masochism, or familiarity with the game of the player. It seems a difficult balance.
Forbidden Island, Pandemic, and Forbidden Desert. All are excellent and have that balance down perfectly. And vary in complexity and theme to suit a range of players. Forbidden Island is a fantastically quick and simple game that feels very much like a race against time as the island sinks beneath your feet. Pandemic is the most complex and probably the most rewarding as a result, it also adds a sense of gravitas - instead of reckless treasure hunters you are now doctors and scientists trying to save the world from disease and bring it back from the brink. Despite loving Forbidden Island I ignored Desert for sometime assuming that it would be basically the same and an unnecessary addition to the collection, however it is more than worthy in it's own right adding a level of complexity and new mechanics. Most notably increasing to 5 players and an additional way to fail with each player needing to keep a canteen of water to survive the ravages of the desert.
Forbidden Desert was introduced on a recent gaming weekend and we failed 5 times in a row and we're quite good at games! More impressive than beating us is the fact that on our defeat we become more determined to beat it. To quote Ross: "With most games I want to play most when I haven't played them for a while, with Pandemic the time I most want to play it when I've just finished a game!" This then is the heart of a successful game (co-op or otherwise) wanting to play it again, and maybe when it's all of us together against the game it's easier to want to try again, there's probably something psychological there, but whatever it is consider me converted!
Arkham Horror. There are a few problems with Arkham - firstly, it's a long game with quite a lot of faff, and secondly the difficult swings so wildly that you can end up playing for hours safe in the knowledge that you have definitely won or lost, because when things go right they go very right and when things go wrong they go really really disastrously horribly wrong! But I suppose that is pretty true to old H.P.'s stories (except the bit about things going right!) So perhaps not the best of co-op games, maybe they need to be shorter or quicker to make failure an acceptable option? But I liked the theme, artwork, and general style so I was willing to give Elder Signs a go, also I'm quite a fan of rolling dice. What I hadn't notice was that it could be played one player! Solo play is not something that has ever really appealed, for me part of game playing is the social aspect, the dynamic back-and-forth and the competitive nature of gaming. But this is a game designed without that player versus player competitiveness in mind, so perhaps the game doesn't suffer (mechanical at least) from a smaller number of players.
Elder Sign feels very like Arkham Horror but with a neater tighter scenario: the sprawling town of Arkham is replaced with the museum, and this shift helps the theme seem less nebulous. The mechanics seem less of a faff and the time invested is reduced, both of these are big wins. It plays really nicely as a solo game - I've played twice with massively differing results - first victory by a narrow squeak and secondly a crushing and brutal defeat. Whether this is symptomatic of unevenness is yet to be seen but I enjoyed both games and the loss does make me want to play again, so time will tell!