Simply Restful in Rails Core - a comment

1. Mappings in routes.rb: url_path(may_contain_regexp) <-> controller . "GET: /users/1" isn't enough, I want "GET: /users/friends/jesse"

You can do this now with /users/jesse/friends

map.resources :users do |user|   user.resources :friends end

No one ever said that resources have to map to a model necessarily, or that the :id param has to be an integer.

2. Ability to implement other http methods (http, webdav, custom [YES, why not?])

I've heard of webdav implementations in rails, it's possible.

3. Arbitrary content types for the resources (XHTML, XML, but also plain text and binary)

You can easily register your own mime types. I believe it's Mime::Type.register...

Though simply restful's design is pretty flexible, you're correct that it won't handle every case out there. It's not meant to. There's no reason you couldn't just write your own routes for whatever restful layout you want. And if you're using it multiple times, make a small plugin out of it. That's how these things start.