The eight cores and 16 threads chip will have better memory support, will be upgraded to DDR4 and the LGA2011 socket gets a tweak, breaking backwards compatibility. But the best bit about appears to the best might have eight cores advertising but will actually have another six hidden.
We have seen this before. Because the CPU dies underpinning both the existing Sandy Bridge-E Core i7s and those upcoming Ivy Bridge-E's actually contain eight cores. Intel just switches two of them off.
According to Techradar the Haswell-E will have 14 cores. But on the desktop we'll only be able to get at eight of them.