If this wasn't an MVC app I'd say make the form get not post. However, perhaps it should be something like
As a work around, you can manually send get variables to the url and it does sort. e.g http://nuget.org/packages?q=&sortOrder=package-download-count
You're right, post doesn't make much sense. Maybe we can fix this.
Our new API v3 takes a different approach.