Default Assignee for Issues without a Category

There isn't a way to specify a default assignee, however, there is a way to automatically notify a set of team members that there is an unassigned issue in the system. You can configure this from the project settings notifications page for each project.

The reason we chose not to have a simple default assignee is that if an issue automatically gets assigned to someone who is on vacation or out of the office for a day, there can be a significant delay in that issue getting the attention that it needs. As a result, instead of enabling a default assignee for the project, we let you set up the afore-mentioned notifications to multiple people on the project so that it can quickly be assigned to the correct person.