Why is my HD full of nothing?
Two things. 


The spotlight process that catalogs files has stopped (perhaps due to low disk space) so they yellow information is incorrect. It should show you a warning IMO that the catalog process failed or needs to update. The "Other" is a catch-all so you're seeing that.
Copy 20 GB of files to an external drive (photo library, movies, virtual machines are easy targets) and then delete them. You can use Finder to show folder sizes and locate some targets. At that point, restart into Recovery and repair the drive and volume using Disk Utility.


Then boot the system normally and let the system run for several hours to see what really is using space. Once you have a good reading, you can add files back from the backup. Your wipe and start over is fine too. Maybe let the system stabilize and show realistic data before restoring your user account and settings/files.

See these for more details:


Why is it important to keep lots of freespace on OSX? What is the impact?
Finder Reports Disk is Full, Other Tools Don&#39;t Account for it