Patrick Logan writes an insightful blog entry about problems with ‘rigid’ languages:

I am not sure why Python is changing or needs to. But the core language is more limited than Lisp or Smalltalk in the sense that it is “C”-like and distinguishes significantly between function calls, operators, and control structures … A more uniform language would not have as many of these backward-compatibility issues. This is the main issue I have with Python… it’s syntax is OK, but not great for extension.

This is also one issue that I have with Haskell (its syntax and semantics aren’t quite malleable enough for some embedding of domain-specific languages), but I’ll save that rant for a rainy day.