Every complication along the way gives the user a chance to stop and rethink their buying decision, while chasing away most impulse buyers among your users. This is obviously less basic than the need for air or food, but it’s probably the key factor to increasing conversion rates.

Main takeaway: Facilitate a simple checkout, avoid forcing the user to create an account or use a password.

As few steps as possible to take an action.

That is the way of the Web.