More bandwith, faster ram, better parts etc. etc.
Actually it comes down to one thing caused by two parts: more layers. The more layer needed for a mobo, the higher the cost and a X79 mobo has more layers due to the two extra memory channes and 4 extra DIMM slots and as well the extra PCIe lanes.
Added features are Quad channel DDR3 vs dual channel DDR3, 40 PCIe 3.0 lanes vs 20 PCIe 2.0 (3.0 if a Gen3 Z68) lanes and more SATA 6GB possible, I think the server boards have like 10 SAS/SATA 6GB/s.