If you were in charge of usability research for StackExchange.com, what would your priorites be for user testing?

Edit: I revised the question after receiving helpful feedback. @Luke Smith - thank you!

Why Use StackExchange.com as the Focus?

I believe that using StackExchange.com itself as the focus will prove advantageous because:

• When you discuss a particular test you would design, we are much more likely to quickly understand the test and see why it's important. (As opposed to discussing another website.)

• A real world example, here-and-now, is much more interesting and engaging, i.e., we will learn more.

A Request

I understand that we should ask one specific question that is likely to yield one or two good answers (sometimes more), and that will not seduce members down the slippy-slidey slope of opinions and emotional debate.

Therefore, please resist the temptation to comment or answer with evaluations (good or bad) of StackExchange.com's current usability or user experience. (Unless it's directly tied to your user testing plan. :O)

Why am I asking this particular question?

My hope is to receive answers that deepen my understanding regarding prioritizing, planning, and implementing user testing projects.

Thanks!