There appears to be no way to get directly back to the "Featured" list of packages, which are shown by default when you first launch Discover. For example, if you click "Installed" near the bottom of the leftmost panel, your only way back to "Featured" to first click one of the top-level categories (e.g., "Applications"), and then click "< Back" at the top of the leftmost panel. I think a "Featured" option at the top of the lower list of the leftmost panel (above "Installed", "Settings", etc.) would probably make sense.
Click the "Home" button next to the search field on the toolbar.