3rd party tools and libraries. I'm having trouble making some things work under 2.6 (not a LOT of trouble, mind you, but some.)
As I see it, the ecosystem is one of Python's biggest strengths, and going to 3.0 at this point chucks a lot of that out the window. (In a year, I'm sure the story will be totally different.)
(Obviously, there's a chicken and egg problem here, but fortunately it's not my problem to solve, so I'll stay with 2.6 for a while.)
