[pjmlp]

The way unicode was handled, integer division semantics, some libraries went away, and a few other things change enough that 2to3 wouldn't handle everything, which in a dynamic language is a big pain to track down in a huge codebase.

Naturally some changes on the C API for native libraries as well.

[Semaphor]

Fair enough, thought that still sounds like things where an upgrade path is available, just not automatically, instead of "you have to throw the code away because it simply doesn’t work anymore".