I think that what actually confuses users is that the default option for general search after they register is not "Open tasks". Whenever they use the search field in the header, they get all those commits mixed with tasks, and they wonder why.

The fact that the customization of the default search is not easy to discover is a problem indeed, but the original problem is that Phabricator's default option seems to confuse many users (in our instance, at least). Admins have no way to change the default for all users.

Just curious, what is your default option for the general task? I have "Open Tasks", also here.

In T7266#96398, @qgil wrote:

I think that what actually confuses users is that the default option for general search after they register is not "Open tasks". Whenever they use the search field in the header, they get all those commits mixed with tasks, and they wonder why.

I have created a task specifically about this problem at T7314: Default Phabricator search criteria confuses new users mixing commits and tasks

I've marked D12509 as fixing this, although it really just obsoletes it. Specifically: it removes this behavior (we no longer default to the first query) and makes scope selection explicit instead (if you want to default to a specific query, just select it from the dropdown).