There is just so much you can learn from customers (or potential customers) that you should spend at least one day a week talking to them. You'll be amazed at what you can learn.
Technical founders especially love dreaming up cool stuff and getting to coding.
This is a huge mistake.
I recently spoke to one of our customers and asked them what their favourite feature was. I was stunned when they mentioned that sending email when a comment was added was a killer feature for them.
The shock was because we always thought this feature as a commodity feature. I mean, every tool out there has the ability to send email when a comment is added. We had a whole lot of really killer features, but this was one of the key features??
So I probed further.
Turns out that other tools only send out emails for discussions that you are involved in. Our tool sends out emails for everything. Think of a mailing list - you get every email whether you participated or not. Whereas a forum only sends you notification for new replies in the threads where you participated. It was something like that.
We always thought sending email for everything was a limitation. It was on our roadmap to refine it and make it send email selectively.
Well, guess what? Rather than being a limitation, it was actually a feature! And a killer feature for them.
So I called another customer - and this was an important differentiating feature for them too!
In a startup, we make a ton of assumptions. Don't forget to get them validated. Usually what the customer thinks is important is not what you assumed it would be.