Re: Workspaces improvements
Yes, Workspaces are a killer feature (made me move to opera).
I suggest to not only change the number of workspaces, but also enable the organisation in a tree (main- and sub- and sub-sub-workspaces ...).

Example:
Workspace 1 - Work/Project
Workspace 2 - Travel - frequently used pages for hotel and flight search
Workspace 2.1 - Trip to Vienna - search results for flights and hotels for further selection
...

There are addons/plugins for other browsers like chrome and firefox for tab management.
For example Workona works good, but the tabs are stored in the cloud and the browser shows a separate window with all tabs which has to be ignored. So an addon has limitaions. Also the cloud storage could be blocked inside a company firewall. Therefore a native implementation in opera would be very useful.

Re: Set default workspace for bookmarks
When designing a default workspace, another setting for external calls could be useful.

I use Opera as default browser in windows.
When I open a link from a mail program or any other source outside of Opera I would like to have the page opened in a certain workspace.