As I learned at the SF Lean Conference and the Lean Startup books, building the right thing is mission critical.
Without achieving good product / market fit, the product is not commercially viable. This concept was reiterated in this excellent post on running a company while working full-time:
Basically, the article poses that much of growing a company is marketing it and finding the right customers for your product.
So as much as it is building the product, it is critically important that it is the right product being built for the right audience.