dynamic expansion in minibuffer no longer works when upgrading from Emacs 21.2

[Android Eve]

Continuing my process of migrating the latest & greatest Emacs 23.2, I
hit another unpleasant surprise: dynamic expansion in minibuffer no
longer works!

By "dynamic expansion in the minibuffer" I mean the feature that lets
you blindly hit the spacebar to complete filenames, variables, etc.

I also invoked 'Emacs -Q' (to rule out any .emacs artifacts) and the
problem exists not only with Emacs 23.2 on Windows XP, but even with
Emacs 22.1 on Ubuntu.

Something has changed in Emacs' default behavior, but what is it?

Thanks.