What are the key features?

Some of this is a bit of repetition from the home page, but it's worth repeating:

• Zen uses the "left-sidebar" and "right-sidebar" for its sidebar regions. Layout Studio uses the more semantically correct "secondary" and "tertiary", since the secondary or tertiary regions could be on either side of the primary content.
• More layout options. Zen can only have a right and/or left sidebar, but cannot have two sidebars to the left or the right of the main content area if your layout requires it.
• More body, block and node classes.
• Random class feature. Ideal for creating variation per browser refresh.
• Simpler installation and easier setup of basic site wireframe.
• Less complex template files/HTML output.
• Built-in "credit link" where you can put your firm's credit text or logo on either the home or all pages.
• Built-in function for copyright information and automatic date span (eg. 2008-09).
• CSS reset and beautiful typographical defaults.