Unleash the Grouchy Person!
The Imaginatik Blog references an interesting article titled Why innovation happens when happy people fight. It covers the need for constructuve conflict, but also the evidence for using humour and optimistic people for creative roles. What I really like though is that it advises you to hire a few grouchy people - they tend to be better at identifying risk and weaknesses. But becasue their misery is contagious you should only let them out of their office at tactical moments. I worked with such a person once - he had a brilliant analytical mind and when something failed we often realised he'd been telling us for 6 months, but to follow his thinking took hours so few people had the patience.... that really got him down.