This here was an addon I used for ages.
http://www.wowinterface.com/download...tatusBars.html
I used it in the WoW classic beta already.
HUDs are the same. Vertical statusbars in the center of your screen giving you information without distracting you from your character.
Additonal functionality and information comes from the standard unit frames in the top left.
Well. That was how it started.