It really depends on what your product is, but ultimately you want to learn about their experience using your product (what obstacles they've come up against, what features they use most/get the most value out of, things they don't understand, etc).
You can also calculate an early NPS (net promoter score) by asking how likely they would be to recommend your product to a friend. (http://www.netpromoter.com/why-net-promoter/know)
It'll be tempting to focus in on the areas that are of greatest concern to you (and you can certainly bring those up) but try to stick to open-ended questions to learn the most from your interviews. ex: Instead of "Would you use a new feature that helps you do X?" ask "How do you imagine you'd use a new feature that helps you do X?" or "What new feature would you want us to add?"