I create a subprocess by dragging the Subprocess Activity node from the palette to the drawing panel. With F2 or by going to the properties, tab "General", I describe the process. The process appears in the Structure view as a folder with the just entered description.
When I click the '+' sign, the node "opens" and the text disappears. I understand the node is now a mini-screen, but the description in the Structure view disappears.
By default, yEd supports different descriptions for opened and closed group nodes. If you do not want that, but require the same description for both states, go to "File" -> "Preferences", tab "Editor" and activate "Synchronize Open and Closed Group Label".
Now I find it a bit clumsy to drag nodes from the palette into the expanded subprocess node. Most of the time I have already drawn part of the process and just want to drag it into the expanded node. Which doesn't work; someone else already brought this up http://yed.yworks.com/support/qa/1794/im-using-yed-3-with-bpmn-how-do-include-things-in-a-sub-process
You are wrong. It is possible to drag existing elements into an opened group node. To do that, press and hold SHIFT then press and hold the left mouse button and drag the selected elements over the open group. When SHIFT dragging elements over an open group, yEd will display a visual indicator if releasing the mouse will assign the moved nodes to the group in question:
Note, when releasing the mouse in the above example both yellow nodes will be added to the group node and the group will automatically grow as needed:
I suspect the disappearing and reappearing of the description has to do with having a different label. Does this also serve a functional purpose, like is there a reasone one would name the collapsed subprocess differently than an expanded one in the Structure view?
Yes, sometimes it is desirable to have different descriptions depending on open/close state. If you do not want or need that feature, enable label synchronization as described above.
And what's the difference between the nodes "Task Activity (Open)" and "Subprocess Activity", manually expanded by clicking the '+' sign?
There is no difference between those two.
I'd like to add: when collapsing the nodes within the nodes, the parent node adjusts its size to meet the needs of the subprocess. But when collapsing to go up in the schema, the parent node keeps taking up the space and the edges are not the same anymore either. I find this very annoying as I'd have to re-layout after a collapse to keep a compact overview. Is it possible to skip the collapsing and go straight to the full screen view?
I do not think I understand that question. What has "collapsing" to do with "going up" and "full screen view"? Moreover, navigating in and out of a group should not change the layout in any way.