Yes, I was able to figure it out, as I mentioned in the OP; however, the issue is that the interface of the Product Ideas space has some counterintuitive UI elements. I apologize that my heading was not more specific in this regard.
Allow me to rephrase the issue:
The landing page for the "Product Ideas" space (Product Ideas) does not have any link or button to submit an idea. The intro text says "You must be logged in to participate." (emphasis original). This message is consistent the UX of a "You must be logged in" message replacing the "Create thread" button when it cannot authenticate the user, and therefore makes it seem as though if I were logged in I should be able to see an inline link or button to create a thread. As an example of this type of UX, on the landing page for the "Discussions" space, there is a bar at the top where you can begin a process that gets you to starting a new discussion, and if you are not logged in, it will prompt you to do so before proceeding to the discussion creation step.
Another common functionality is to have the "Create thread" button at the end of the "Complete guidelines" page (About the Product Ideas discussion space), which is linked inline at the bottom of the introductory paragraph. However, the page linked at "Complete Guidelines" ends with a link back to the Product Ideas page.
I had been able to submit an idea by going to the "Content" tab of the Product Ideas space and clicking the "Submit an idea" link on the left.
It is only now after looking at all of these pages many times that I see there is a "create an idea" link on the right, at the end of a list to "Create by e-mail" and "View feeds". It would be easier to notice the "create an idea" link if it were more prominent, i.e. by being first in that list, as is the case on the Report a Product Issue page. Other spaces have fewer items in the right sidebar so it is easier to see their respective create-a-discussion links. For example, this is the only space landing page of the three that has its own "Search" widget, however since the heading and button label are generic, it is not apparent that the functionality is different from the search bar at the top of the page.
Therefore my specific suggestions for improvements would be:
- Move the "Create an idea" link on the right sidebar to the top of the three items in the list where it resides;
- Add text to the intro section of the Product Ideas page and/or to the "Complete Guidelines" page that directs the user on how to submit an idea, e.g. "To create a new idea, click the "Create an Idea" link at the right."
- Change the "Search Widget" heading or the "Search the Community" button label to reflect the fact that it only shows results from the product ideas space (e.g. "Search Product Ideas" for both heading and button label).