Confluence is used to store a lot of types of documentation, including project specific information for current and historic projects, product information, discussions, etc.
It would be ideal if the quick search would perform a global search but ignore some spaces that contain historic project information and/or discussions, as most often people need to find the latest information from product specific space or the most current project.
If this is implemented than there should probably be an option to perform global search including the filtered out spaces.

This would help to control the number of hits a search returns and show the most relevant entries.