Planning is important, and cautiously thinking about your features before diving in can save you weeks later on. But spending 6 month in a tunnel, writing specs you'll spend at least a year to implement, is a huge risk : what if your users only use 30% of the features you designed, and lack a missing feature you didn't plan ? You could have figured it out way sooner have you built something earlier and released it.

Also, your time to market is way slower and you don't risk losing years of work by missing a deadline …

> Damn, now I need to move faster. My app must be ready by the time they close Wunderlist. I expected an eventual closure, but not that soon.