Why use Aquamacs instead of the a stock Carbon or X11 Emacs on the Mac? Here are some features that Aquamacs has to offer on top of all the advantages that come with Emacs:
Fonts just work, right from the menu: The Mac-standard font (Lucida Grande) is the default for editing text, and the mono-spaced Monaco is used to other modes. These fonts are nicely rendered with antialiasing. Aquamacs offers a range of proportional and mono-spaced fonts to choose from.
Aquamacs Emacs has a standard Mac menu with entries where you would expect them, and recently used files are available from the File menu.
Aquamacs Emacs gives you all the standard Mac shortcuts like Apple-S, Apple-C, Apple-V - everything that you're used to.
Aquamacs Emacs can open a normal OS X window for each file that is opened - Emacs experts call such windows frames. Finally, Aquamacs Emacs makes use of the capabilities of windows on modern graphical user interfaces. This is configurable with a mouse-click - of course, You can switch between the windows (frames) with the "Buffers" menu.
Perfect Team-play. Clipboard operations interoperate with other Mac apps. In Aquamacs Emacs, if you mark a chunk of text, it will NOT be automatically copied into the clipboard. Just like in any Mac program, you can copy with the Apple-C command, and then replace another region by selecting it and hitting Apple-V (Paste).
Support for Asian languages. In addition to the input methods provided by any current Emacs for OS X, the Aquamacs Multilingual Environment defines configuration settings for Chinese, Japanese and Korean on top of the standard Mule. A font-package is included (not preactivated).
Printing just works the way you expect it to - with a preview and through the normal printing infrastructure.
Meta Key Management: since Emacs uses an extra modifier key that is not on your keyboard, Aquamacs allows you to use the Option key (or others with configuration). But even then you won't lose the ability to input characters such as [ or \ with the most common non-English keyboard layouts.
Frame Appearance Themes: You can define fonts and colors as defaults for a given editing mode. That means that you can have customized designs for different types of files you're editing. For example, you can easily distinguish LaTeX files on the screen by their beige background color, if you pick a color theme for the a latex window and select "use current theme for latex-mode".
A number of little extensions specific to the Mac are contained - they're small details that make your life easier. For example, there is a "Show (file) in Finder" function, or another one to open new files in one of many popular modes. When you double-click a file written in Aquamacs, it'll open in Aquamacs (thanks to Creator meta-information in files). You get a menu with recently edited files without any configuration. Of course, we support OS X functionality such as the little button in the upper right corner of windows that lets you switch the local tool-bar on and off!
It's a distribution that comes with the latest goodies pre-installed so you can simply use them and get on with your life. No installation, no setup needed. Here's what's included: AUCTeX for comfortable LaTeX editing, ESS (Emacs speaks Statistics) as an interface to statistics applications such as R, SPLUS etc., nXML for comfortable XML editing, SLIME (from Aquamacs 0.9.9d) for superb Lisp programming. Other, smaller, packages provide support for a range of programming languages and other formalisms.
Aquamacs Emacs remains extensible, so you can use special syntax coloring setups or enjoy embedded CVS support, a HTML markup menu and the like. Aquamacs is compatible with GNU Emacs.
Aquamacs Emacs offers a dedicated manual plus the good old Emacs manual directly via Apple Help environment - you can search both of them quickly with Spotlight and read the documentation comfortably.
Great Support. A community around Aquamacs Emacs (and a larger one around Emacs!) will help you out when you don't know how to do X. Just write to the mailing list (but check the manuals first!)
Aquamacs additionally activates a number of packages that are provided with the default emacs, such as cua-base, ibuffer, recentf. The GNU Emacs code-base is patched. Please see the CVS for the collection of patches and additional source-files.