For me, retro means style and construction.
My dad had a 386 in a massive desktop case that felt like it was made out of recycled battleships. The cheap mATX case I bought for my home server feels like it was made from recycled beer cans.
Retro styling means clean lines, not neccessarily sharp edges and corners, but clean, simple design. Cases these days tend to have faceplates and outer panels that look like they were designed by an impatient 8 year old on speed; ribbing and puffed out contours, fan blisters that look like they were taken off of a Star Destroyer and glued onto the shitty thin rolled steel sheet it calls a side.
Granted, most of the well engineered ugly modern cases make their forms functional for airflow and such, but still...hideous.
The Englight case I picked up is almost identical to the one a few posts above, except that the space where the USB ports on the bottom are, are blank on mine.