A big part of the guiding philosophy for how I work revolves around the concept of "fun". Sure, creativity, passion, drive and actually delivering are all part of a job well done, but if I don't feel I have the license to have fun, good ole fashioned feel-like-I'm-a-5-year-old-again fun, then my overall motivation suffers quite a bit.
I imagine the same is true for our end users. Fair enough, they signed up for a 2 hour usability test, but who says they can't have fun in the process? And what of our clients - I know that it's often a very serious affair for them, with jobs in the balance, nay, lives on the line (or it would seem, anyway), but why can't they get to play as well? "Putting the user first" is all very well as the governing maxim of User Experience, but if it means at the expense of my team and my clients - well, I'll have none of it.
Fun, I think, should be for everyone.