Aim for feature parity with Windows Home Server. Aim for lower price.
Need to spend more time on differentiating the exact goals. WHS has many more features than a Drobo and yet even getting Drobo-level functionality is tough with Ubuntu (specifically hot-swap drives and online resizing). Try to explain ‘resize2fs’ syntax to your parents
One advantage over Drobo is the FS size limitations: http://www.drobospace.com/blog/entry/11516/Q-amp-A-About-Using-Drobo-s-Large-Volume-Sizes/
Another advantage over Drobo is performance.
An advantage over WHS? That’s a tough one. However, WHS is not designed for use with Ubuntu desktops. So it’s more like “WHS is great given everyone is using Windows.” Just like Time Machine and Time Capsule are awesome if you’re already using a Mac. UHS needs to be great _given_ that everyone is using an Ubuntu machine already. Centralized file store, centralized backups are key.