How can I customize the Unity top panel?

The top panel isn't empty. It is used for applications menus and titlebars, and you can add many indicators which also requires space. I sometimes like to use my screens narrow and tall, and then there would really not be any room for anything more.

You can't add a taskbar to the top panel. Adding a bottom panel with a taskbar is fairly easy, though. You can use Gnome Panel (the panels from earlier versions of Ubuntu -- they are still available), Xfces panel, LXDEs panel or any other panel you like. There will be no conflict with Unity as long as you only use a bottom panel. In the case of Gnome Panel, you'd have to delete the top panel manually, either by pressing alt and right-clicking the top panel, or you could use dconf-editor from the dconf-tools package.

For instance, this is Unity with an xfce4-panel taskbar, with extras. As you can see, there's no conflict. I would probably recommend that you use Gnome Panel if that's what you're used to. Just remember that in Gnome Panel 3, you have to press alt to display the right-click menu on the panel or move applets. Otherwise, it's just the same as in earlier versions.