Per Xah Lee (search for 2012-11-28), I’ve experimentally turned on icomplete-mode. I’m not sure how I like it so I haven’t included it in my init.el file yet.
According the internal documentation, icomplete-mode “implements a more fine-grained minibuffer completion feedback scheme.” You can toggle it with 【Meta+x】 icomplete-mode or turn it on/off unconditionally with a positive/non-positive prefix argument.
Lee says it’s part of Emacs 24.2 but the source file indicates it’s been around a lot longer. If you have an older version of Emacs and it works for you (or you get documentation for it with 【Ctrl+h f】 icomplete-mode) leave a comment so that others will know.