The insane lead time of Perl 6 to even get to a point of backwards incompatibility was it for me.
I'd started on an early version of perl 4 and went through the 5 transition and was excited about 6. For what seemed like "Duke Nukem Forever" time, and finally my fickleness drew me to other languages and frameworks.
Fun fact: The amount of time it took Perl 6 to come out after being announced was actually longer than the amount of time it took Duke Nukem Forever to come out after being announced
You run the risk of killing the new in the cradle, though, and often that scares people.
from __future__ import coolshit
that's the way you wanna roll