Design teams often use themselves as model users, but they are almost the perfect storm of differences between themselves and the users.

• They know the product exists and what it is supposed to do.
• They understand the internal concept, including its past and future ideas.
• They care, personally, about the product. Their success depends on it.

The user has none of these things. The user knows nothing about the product, doesn’t understand the concept, and doesn’t care.

http://boxesandarrows.com/are-your-users-s-t-u-p-i-d/