I wrote the first version of Gmail in one day. It was not very impressive. All I did was stuff my own email into the Google Groups (Usenet) indexing engine. I sent it out to a few people for feedback, and they said that it was somewhat useful, but it would be better if it searched over their email instead of mine. That was version two. After I released that people started wanting the ability to respond to email as well. That was version three. That process went on for a couple of years inside of Google before we released to the world.
And even when it was released, Gmail stayed in a closed beta for more than a year. After almost four years since the release, Gmail constantly adds new features and it's still in beta ("There's no good reason in the world for Gmail to still have the beta tag. It was supposed to have gone away a long time ago," says Paul).
Paul Buchheit, who left Google and currently works for a startup with other ex-Googlers, thinks it's important to release applications in an incipient phase to get feedback from users.
So what's the right attitude? Humility. It doesn't matter how smart and successful and qualified you are, you simply don't know what you're doing. (...) What is the humble approach to product design? Pay attention. Notice which things are working and which aren't. Experiment and iterate. Question your assumptions. Remember that you are wrong about a lot of things. Watch for the signals. Lose your technical and design snobbery.
Gmail got a delete button after many months of requests from users, even if Gmail's philosophy was "archive, don't delete". Gmail will also add some functionality from folders to its labels, most likely drag and drop.
The key step is to build a product that's interesting enough to a attract an audience and learn from people who use the product. "The sooner you can start testing your ideas, the sooner you can start fixing them," explains Paul.