You aren't as likely to discover their needs by analyzing their reactions to something that you've built. I agree with codeinthehole - research that independently of your ideas.
Some questions that might be helpful:
What's a random day in their life like? When / where do they use tools similar to yours?
What do they need to know beforehand in order to achieve their goal?
How might their needs or behavior change depending on the situation?
How do they currently think about the activities they do with the tools they use now?
(Exa.Ex: Do they understand the underlying concepts or have some bizarre idea about how the technology works? If they don't understand how it works, is there a way you can design it so that it's closer to their imagined idea than reality?)
- What do they know about the tools they already use?
People got all excited about Google+ circles. They wrote long blog posts about the innovative new circles feature. However, Facebook had the lists feature for years. Few people used it, partly because they didn't know it existed, partly because they thought it was too much effort or didn't understand it, partly because of its design.
- What do they like / dislike about the tools they already use?
