So many essential points here that it's impossible for me to pick just one or two. So I'll expand on your point about the importance of a critical eye. Most writers put on their viewer hat when they watch a movie. What writers should watch (and this is difficult on a first viewing) is how they feel when they watch a movie. In other words, writers should monitor themselves when they watch a movie ("now I'm feeling this, now I'm feeling that" etc.). They should "watch" their watching experience, not just the movie itself. Again, this is easier to do on a second or third viewing or reading. You can train yourself to do this, however, but shouldn't do so on first blush. The self-conscious activity of watching yourself when you see a movie / read a script for the first time can spoil the fun, which is part of the viewing / reading experience, and thus misses the boat. It's even harder to watch your experience as you read your own script--but this is what enables writers to improve the re-write stage.

Speaking of which, you probably know the old Mitch Hedberg joke: "I wrote a script, and I gave it to a guy who reads scripts, and he read it, and he liked it, but he said he thinks I ought to re-write it. I said, "F*** that - I'll just make a copy!' "