I started with *nix and a command-line interface for two reasons: 1. This is what I know, and 2. a CLI can be far more easily used as a component of a larger system.
I'm following a fairly standard startup course here: Build something people want, then build something more people want. I expect a GUI to happen in the future (probably using Qt). I expect a Windows version to happen in the future. I expect tarsnap to be included in web server management interfaces in the future. The name might change in the future, or "tarsnap" might just end up being one of the names like "google" which start out looking dumb but end up sounding normal.
But that's all "easy" stuff -- the place to start is with a solid and secure foundation, which is what I'm doing.