Many professional features are included in Mercury
Table-less & CSS-based
Adjustable layout
xHTML & CSS validation
Cross browser compatible - Support Chrome, IE7+, Firefox 2+, Safari, Opera
SEO friendly with SEO parameters
Flash-Free (easily browsered on mobile devices)
Compatible with Joomla 1.5, 1.6, 1.7 and 2.5
Many build-in Modules for smarter and more effective content presenting
Simple CSS-based customization
Large Typography Gallery (bullet list, dialog, icons, buttons, modules )
6 colors
With up to 15 positions: Enough positions for all your needs
Source: ByJoomla[dot]com