A sideways style lends itself well to designs with a bit of an "arty" feel: brochure-style sites, galleries, portfolios, and perhaps even sites that make use of interesting info-graphics or charts. Successful horizontal layouts often use animation and graphics to highlight their sideways movement - a large number of these sites use colorful images and nice, big backgrounds that really show this off. Unusual also means unfamiliar, so a well-designed horizontal layout will also make good use of navigation elements such as menus, arrows, and buttons to overcome any usability hitches.