Read texts, edit them and write documents in the word processor supporting files with complex structures including books, manuscripts, dissertations, etc. Import of old-style fonts into files with modern formats is possible. Writing bi-directional texts is possible.
Mellel is a word-processor for Mac OS X built for long texts. In other words, it was specifically designed to help people who need to create long text documents. Thus, it has features and the design that suits those users best. On their website, the developers of Mellel list a few of the uses this app might help for. Those uses include research and education, creative and technical writing, multilingual documents, and well, writing in general. For each of the uses I just mentioned, there is a list of relevant features. Most of those features, you will probably find in other word-processors, though.
My first impression of the user interface was that it lets you focus on writing. Thus, the user interface is very simple and it doesn't use any bright colors. There are two elements that make up the user interface. The actual page and the controls, where you type and preview your text, and a control panel with an array of different settings.
All the features that I am accustomed to seeing in word processors were there, including track changes, table support, bibliography, styles, etc. You can of course easily type and change your text and insert images, but if you are going to pay for this app, you might as well buy it for one of the advanced features.