Mickey Petersen has written up an excellent round-up of the latest features of the Emacs 24.4 release.
Read the whole article at http://www.masteringemacs.org/articles/2013/12/29/whats-new-in-emacs-24-4/.
He had also written a similar article for Emacs 24.3 (current stable version), which can be found here.
New Settings that I am using
I have already setup the load-prefer-newer, cycle-spacing, and ns-use-srgb-colorspace options.
The new electric-pair-mode options also look interesting, and I have set these up (I was using skeleton-pair for the auto-pairing function till now).
The new M-s . (isearch-forward-symbol-at-point) is going to be very useful. It essentially replicates what C-s C-w does, but having a single key-stroke is a little easier. A closely related command M-s h . (highlight-symbol-at-point) also looks to be useful. This is especially handy with the hi-lock-auto-select-face option.
C-x SPC (rectangle-mark-mode) finally allows replacing CUA for me.
Eshell now has even better support for visual commands; i.e., commands which expect a terminal. See the options eshell-visual-commands, eshell-visual-subcommands and eshell-visual-options for details.
New Interesting Modes
The new Web Browser for Emacs (eww) is great! I have already uninstalledw3m and using eww solely right now.
The dired-hide-details-mode is also nice. It has already replaced dired-details for me.
superword-mode looks to be useful (especially for programming Ruby, where underscores are usually used to separate words in an identifier).
The new file notification system would be very useful, except that it does not (yet) work in OSX (bummer).
Other Default Settings of Interest
desktop-auto-save-timeout is going to be a life-saver. The related option for desktop-restore-frames finally does what I want.
The new electric-pair settings electric-pair-preserve-balance, electric-pair-delete-adjacent-pairs and electric-pair-open-newline-between-pairs do exactly what is expected.
Ruby-mode has some interesting updates, especially for indentation.