If you have to validate email addresses, which I believe you do, what I suggested doesn't add any more steps so I doubt it would cost anything. To the contrary, given that the initial user signup is so simple (just an email and password), users will be much more likely to complete it compared to other signup forms that require more data. Once they are logged in and on your site, you can gamify whatever portions of the profile you need filled, that is, if you need any more data (most sites like hackernews don't).
Actually I'm wrong about hackernews, you'd need a username, but still, you can choose that after you sign up, and you shouldn't be able to log in with that username, that should only be a public handle for your account on forums, comments, etc.
