Frequently Anticipated Criticisms
Markdown isn't a good language for documentation. People should learn reStructuredText and be happy.
The worst kind of documentation is no documentation. If you want more docs in the world, you need to make it more likely that they will be written. Removing a barrier to doing that (like learning/remembering a markup language) cannot possibly be a bad thing. Many, many OSS projects are one person's small gift to the community; it is ungrateful of us to tell them to suck it up and learn a new tool for our benefit.
Markdown can be just as extensible and rich as reStructuredText given the right syntax for extensions. Don't dismiss it until you see a direct comparison. And even if it's a little worse, see last paragraph.
And finally, it's okay if this project is not for you. Other people will do what they like.
You should have just worked on Sphinx
Sphinx is a fundamental piece of the Python community. This is great for the community, but it also means that radically changing Sphinx could be both difficult and harmful.
I think Git and Subversion are an analogous pair of projects to Computer Words and Sphinx. Subversion exists and still works great, but Git introduced some new ideas and a lot of people made the switch because they saw a benefit.
Whether Computer Words has a similar benefit over Sphinx in the long term remains to be seen. My point is that it is still productive to start a new project even when there is already an established one.