Hardware compatibility, ease of use, the size of a software repository. These three attributes are unique to each Linux distribution. But at the same time, each Linux distribution is at liberty to take and mix whatever it wants from any other.
This creates a rather unique situation, where good ideas quickly spread, and bad ones fail. And as a result, there are dozens of distribution updates each month, hundreds each year, in a race to leap-frog each other in the race to the top of the DistroWatch.com charts.
Read it at http://www.techradar.com/news/operating-systems/software/10-best-linux-distros-for-2011-704584