What are the services that publishers provide that we really value? ... Is it thoughtful editing and careful editing? Maybe, but many people accustomed to reading lightly-edited posts on the web may think that this sort of production is overvalued. This gives me an idea: use the readers as editors. For every mistake found, offer $0.X rebate off their next book by the author.
This is an interesting idea, but many people don't realize that editing a book is more than just "finding mistakes." That's a proof reader's job. A good developmental editor helps shape a book from the ground up, and can make a significant impact on the final product.
(Disclaimer: I edit books for a living.)