How can I get an icon of the mounted devices on the desktop, like in Gnome? Or even mount the devices on a folder in the desktop folder.
I have the desktop set to Folder View.

Unfortunately, the "media:/" KIO slave that would have enabled such functionality is not available at this time. Thus this feature is not available.

There are several ways to do this.

First, you can put the device notifier or device manager widget directly on your desktop, in which case it will show a list of plugged in devices, although you will still get a list of actions for the device when you click on it.

As an alternative, if you want to always open the devices as folders and you have Lancelot installed, you can use that. Add lancelot to your panel or desktop, then drag the title of the "Removable" section under Computer to the desktop and drop it there. It will make a new plasmoid with a list of removable devices. You can also drag other lancelot components, like the "Fixed" or "Places" components, and either drop them on the desktop to make new widgets or drop them on an existing lancelot widget to add them to it. Note that the "Removable" section is only visible when you have a removable device plugged in, so make sure you plug something in first. This is the approach I use, actually.

That is one of the advantages of plasma, widgets know whether they are on the desktop or the panel. So the device notifier acts as a button on the panel that pops up a list when clicked, while on the desktop it just shows the list. Same with Lancelot and Lancelot Parts.