The default finder interface has an area on the left with your hard drives, and your common locations, like your home directory, your apps directory, your downloads directory, easily accessible.

I think this can be turned off, and I think that at some point you turned it off... that's the only thing I can think of because getting to the root of a drive just requires going to the left of the window and clicking on the drive... most of the time (with the default finder configuration.)