If talking to your clients mostly gets you new feature requests, it either means your product isn't adequate yet or you're not asking the right questions. In a nutshell you want to get of feel of:

- What was going on in your clients' heads before they found you. What precise problem were they trying to solve. This'll give you insights on whether or not your product is adequate, and on how to phrase your landing page.

- How they went about to look for you. What google search they did, whether they checked out reviews and how, etc. this is to better focus your SEO and adwords campaigns.

- What their decision process looked like. Was it a single person making the call, or were several people involved? What objections got raised? What were their key decision criteria? Raise them in your sales copy or in drip emails.

- Are they talking about your product? Would they? This is to help you spot segments with viral potential. Don't forget to politely ask for referrals if applicable.

- And, of course, feedback on the product itself. But don't spend too much time on this, and don't promise anything or build expectations.