Great question. And fwiw, we love Express too! After building a number of Express applications myself, I quite often ran into organizational issues (such is the loosey goosey way of Sinatra). Matador takes more of a Rails approach to organizing your code; of course, without all the extra cruft that comes with Rails.
We still tack on Express requests and responses onto your controller instances.
Most of all, the router -> controller mapping with request filtering (middleware-like) is where Matador comes in most useful. We have other features we'd like to add, especially in the scaffolding layer, but figured we'd open source early and get developer feedback.