In your experience, what are the most important questions to ask in a survey for a product redesign?

Direct interaction with users is prohibited by my organization, but I have been allowed to conduct a simple survey by email to identify usability issues. I'm pretty early in the process, so there's still a lot of room for things to evolve.

I remember reading something about this, but I can't remember where or find anything in my notes. (About face? Card Sorting?)

I would like to keep things short; probably < 10 questions. What have you found to be most useful?