Have you ever heard of the the Dunning–Kruger effect? Nathan Bransford just blogged about it, and I found the post completely fascinating. In brief, the effect explains why all those completely tone-deaf singers on American Idol are convinced they're going to win, while the best singers suffer from a deep-seated conviction they don't really belong on stage.
The same effect works on our ability to judge how well we write. I don't pretend to know anything about cognitive bias, but I do know that the more I spend time with writers, the more I am amazed by the talent that is out there. When I was at SCBWI New York this past January, I was astonished at the creativity of the work submitted for critique, and I'm blown away by some of the entries I'm in the process of judging for the Sandy Contest right now. I suspect most of those writers would say they didn't have much talent.
If you consider it, maybe it's not so much about cognitive bias as it about what Socrates observed so long ago: "The only true wisdom is in knowing you know nothing."
The take away? If you feel great about your writing, go and read a great book. If you believe your writing needs improvement, keep working--you're probably right on track.