One of the biggest differences ( in importance, not size) is ECC memory. A recent study by Google - http://www.theinquirer.net/inquirer/news/1558338/memory-errors-frequent - showed that memory errors can be much more pervasive than previously thought. If you are designing something critical, the right answer can make all the difference. ECC Memory is one of the key differentiators between high end gaming box, and mission critical design workstation.
That was one difference that popped into my head as well, I can understand this in an IPDC such as google or any larger cluster compute environment, but what role would it have with typical workstation usage? Is this the only advantage of Workstation over High End Desktop?