Rob Pardo is VP of game design at Blizzard; he led the design team on World of Warcraft. Gamasutra has a snippet from him on game design, learned from watching his 7 year old daughter (a level 54 Warlock) play videogames:
Playing games with my daughter is great because it really allows me to see what is fundamentally fun about game mechanics. For example, when she was only 5 years old, she would just jump off buildings over and over and collect lots of different outfits for her character.
I generally have more of an "achiever" mentality when I play games, and playing with my daughter really helps give me a more rounded perspective towards games.
Interesting that both the guys interviewed had a "goal-oriented" style of play, perhaps a very adult, masculine style, and by watching their kids playing they learned to apply some more exploratory mechanics.
It's really cute that the only difference in their setup there is that Rob has a traditional corporate gel wristmat, and his daughter has a cartoony (winnie the pooh?) mat. Bet she kicks her dad's ass in pvp.