I like the analogy that a finished first draft is like a freshly baked pie. When you first take it out of the oven, you have to put it on the rack to cool. If you dig into it right away, it falls apart, the insides come oozing out as you attempt to put a piece on your plate. And if you try tasting it, your mouth gets burned.
In my experience, gaining a bit of distance from a very first draft is essential. The degree of objectivity gained with even a few days of “cooling off” helps enormously as you go back to appraise what you’ve come up with. During this time find some way to engage yourself in another all-consuming project, perhaps even start working on another new script idea. Or as Marsha Norman explained to me, “You should just find wonderful things to read between the time you put the play away and the next time you pick it up. You should fill your mind up with other language, other characters’ concerns.” However you do this, the idea is to get some distance, get the project out of your head as much as you possibly can.
During this breather period, it’s also critical that the script remains a private experience. Your script is still incubating. It’s a very delicate phase and this is not yet the time to start sharing your initial draft with anyone. Getting input from others now could forever destroy that special, intimate relationship you’ve been nurturing with your work and that you still have a use for.
The point here is that your ability to judge a first draft’s merits can only be trusted if you keep it to yourself and allow yourself to gain some objectivity. And the only way you can achieve that is to put it on your own private rack for awhile and walk out of the kitchen. I’ve never encountered an exception to this.