"So, you switch to workspace 5 and start a long running task. Then you forget about it and go back to whatever you were doing before. What tells you that there is a long running task?"
I never really run into this problem at all since I have always used alt+tab to switch between windows and GNOME Shell shows the windows from all workspaces. I had to use the workspace switched in GNOME 2 because alt+tab in GNOME 2 didn't behave that way.
On the other hand, when I deployed GNOME 2 on a large scale, users who accidentally moved the windows in a different workspace never really found it and the very tiny indicator in the workspace switcher didn't help at all. They would launch multiple instances and complain that Linux was slow.