Unicode support

Emacs 23 and earlier displayed empty blocks for characters when no suitable font was found.

Emacs 24 shows the hex code in the block. From [C-h n RET] (or M-x view-emacs-news):

"Enhanced support for characters with no glyphs in available fonts. If a character has no glyphs in any of the available fonts, Emacs normally displays it either as a hexadecimal code in a box or as a thin 1-pixel space. In addition to these two methods, Emacs can display these characters as empty box, as an acronym, or not display them at all. To change how these characters are displayed, customize the variable `glyphless-char-display-control'."

Useful Keybindings

[C-x 8 RET] (or M-x ucs-insert)

Use this to input a unicode charcter without resorting to any of the Input methods. For example, to insert a snowman type [C-x 8 RET SNOWMAN RET] or type [C-x 8 RET 2603 RET].

M-x customize-variable RET glyphless-char-display-control

Control how characters that don't have glyphs are displayed. Typically glyphless characters are rendered as rectangular boxes with hex numbers inside them.

(From NEWS file) - "Enhanced support for characters with no glyphs in available fonts. If a character has no glyphs in any of the available fonts, Emacs normally displays it either as a hexadecimal code in a box or as a thin 1-pixel space. In addition to these two methods, Emacs can display these characters as empty box, as an acronym, or not display them at all. To change how these characters are displayed, customize the variable `glyphless-char-display-control'."