Standard Ubuntu user accounts are not on sambashare group by default, thus can't create shares

That's it.

When you add a non-admin user account, it can't create a share until you add it manually, using an admin account, to the sambashare user group.

When trying to create a share using Nautilus as a standard user (non-admin) you'll get a very descriptive error message printed right into the window. That is, descriptive for somebody with a lot of *nix XP, otherwise completely bewildering.