Python has 5 implementations listed on their homepage:
- Python / CPython
- Stackless Python*
When considered against languages such as PHP or even Java that seems like a lot of variety not seen elsewhere. Yes there's different compilers available for C/C++ but for a popular interpreted language this variety seems unique.
It would appear many of the implementations are still on the 2.7.x branch, so I might surmise that this has slowed adoption of the 3.0.x series (along with backwards incompatible changes).
So, how has having multiple implementations affected Python as a language?
 Stackless python is a branch, not a different implementation, but you see what I'm going for