> even though javascript has something equivalent to python's setattr().

Python provides significantly more hooks into the behaviour of arbitrary objects, and arguably provides more flexibility than JS.

Here are a few notes from Mike Pall (LuaJIT) on Python features making a straight interpreter slower, and the language harder to optimise (JIT tightly): http://lua-users.org/lists/lua-l/2004-11/msg00083.html

Animats is full of shit, the ability to add arbitrary "static" attributes to existing objects is really not the main problem.