There are two kinds of first drafts: bad and incomplete. It is possible to be both. It is impossible to be neither. (I have heard rumors that some writers write very “clean” first drafts that hardly have a word out of place. I believe these writers either are lying or are aliens.)
The first draft is supposed to get your ideas out onto the page, for you to rearrange, edit, cut, add, and massage until you think it’s good. Then you send it to an editor and you’ll realize you were wrong. But it very nearly was good. And the editor can make it good.
Don’t strive for a good first draft. Strive for a finished first draft. After all, it’s impossible to edit a blank page.
I answered this question on Quora, where the text above first appeared.