Moodle themes are many and varied and can include extra functionality as well as styling which highlights your brand. Here are a few details of themes I've used and why they were chosen..
A highly graphic and image-based them in bold colours for a company running safety courses for industry/construction. I designed this site from the ground up using a theme called Fordson which is highly customizable. The course lists are particularly flexible allowing users to see their progress at a glance. Ideal for busy people.
This is a site I use for open courses including my own. Most of them are self access and open to guests. They show some aspects of course development and my own Moodle plugins. The theme is a basic Moodle one but with a customised front page insert. The page shown is the list of open courses.
Unless there are strong reasons for departing from the basic Moodle themes it is beneficial for sites to stick with them as there will be no associated extra costs of maintenance and development. The site was built for Physical Training course which, by their nature, are taught partyly online and partly face to face (blended learning). A straightforward design where participants will see their courses right away and know something about the courses and the tutors.
In this design the courses could be shown before users are logged on so that visitors can view the courses available before being registered.
While Moodle courses all have common characteristics which make them immediately recognisable, there are still some things to know about design and develppment of courses. Use of small images and the use of whitespace can make them easier on the eye. A good variety of activities are available including interactive elements using products like Articulate Storyline (you can also see examples of the linked from the front page of the aite).
Video is often the preferred method of communication these days and I also produce these using software such as Camtasia to supplement Moodle courses.