Not very. Why? When you think about what a first draft is for, the relative unimportance of it becomes apparent.
The first draft is about getting your story down in an organised format, allowing the characters to breathe the air of the storyline and to give you something solid to work with.
It is NOT the final version you send to an agent/publisher or even to your beta readers, nor is it something you will ever look back on and say 'WOW' about. It is nothing more or less than the first step, if you're a pantser like me, or the second step if you plan and organise your story.
In the first draft we can make spelling mistakes, grammatical errors, leave small plot holes, unfinished dialogue, note places where more research is needed etc. We don't have to worry about fleshing out the characters too much but can learn a lot about the story, the people and world we've created.
The first draft is for us. It shows us things what no amount of research or planning can bring out and helps define whether the story is workable or not. It is not something to fret over, worry over or obsess over.
Our fist attempt to write is not our last and does not need to be perfect or even very good, it just needs to set out the signposts and the general direction.
How important is the first draft? In the grand scheme of writing a novel, not very. It is a stage along the way, not the final destination.