Thanks for your work, this type of application is much needed.

I am curious about the backend architecture - if it is modulary, how difficult is it to write a plugin for a new filehosting site?
Can plugins from some current projects be used? For example, I would like to see support for hosting site 'uloz.to', but It has already been implemented in both JDownloader (you mentioned it as an inspiration) and PyLoad (http://pyload.org/). Other modules are provided by, for example, CLI app Plowshare (http://code.google.com/p/plowshare/). It would be nice to use these modules instead of reinventing the wheel.