I've read that and Joel has a point.
However, I often see (even in my own software) features that NOBODY uses. And certainly, no one chooses us because of those. And they are equally as many solutions that thrived because they were simpler than existing bloatware. I'd argue Trello's simplicity is a Feature not a negative.
I HIGHLY recommend the book What Customers Want.
It's REALLY easy to think you know what your customer want, even if you talk to them extensively. For example they say "easy to use". What EXACTLY does that mean?
If I'd asked what they wanted, they'd have said "A Faster Horse" - Henry Ford
Customers don't know what SOLUTION they need (until they see it) but they DO understand their problem. And they'd rather talk to you about THEIR problem than YOUR solution.
**Talks to customers. Lots of customers. And ask the right questions. And find some way to assess what the value will be to them. "How much are they spending on the current solution", etc.