When it comes to Usability, there is always a lingering question for me:
How do you know you aren't just building a self-serving Web site?
Surely, there will always be biases by the information designer or the creative studio as to what is the simplest and most engaging design. The goal, of course, is to build a site that an average user can successfully navigate. And an average user would never have the detailed knowledge of a Web site that the information designer has.
A common solution involves the use of "personas." You create a fictionalized customer with a personality and a list of goals. Then you pretend to be this persona and try to make use of the Web site you are designing. The idea is that accessing things from a very specific agenda will give you new insight into the usability of your site and allow you to make changes to accommodate your persona.
But does this really work? I was reading a blog entry by Mark Hurst recently ('On product management, personas, and customer focus'). He points out that nothing can replace actually speaking with your customers (or at least listening to their feedback). And I have to agree with him.
There are, in my opinion, two critical issues with "persona" based design. The first one is choosing the right personas. If you pick based on your intended demographic instead of your actual demographic, then you aren't looking at your average user. It becomes a matter of your biases being migrated from your design beliefs to your persona beliefs.
And suppose you do successfully get your personas right. The second issue is your ability as an actor. The underlying assumption has to be that the persona is someone other than you. So how well you are able to get into someone else's head and behave as they would becomes the challenge. And even good actors don't always envision the role the same way.
Bottom line is that creating a good user experience must always start with the customer. Then it is the role of a good designer to take their feedback and channel it into a good design. The customer IS always right in Web site design.