My "workspace" folder includes a hidden folder called ".metadata". Is it important to backup this folder regularly?
The .metadata folder is an area where Eclipse plug-ins can save any information they want to save. This will include things like your preference settings, information about the contents of projects and the indices the Java plug-in uses to find classes and methods quickly.
It also includes the .log file where Eclipse logs errors.
Exactly what is stored there depends on the plug-ins you have installed (one of my plug-ins has a database in the .metadata) so it is a good idea to back it up (along with the projects).