http://www.magentocommerce.com/blog/comments/magento-community-edition-one-eight-now-available/

Enhanced Tax Calculations
Magento Community Edition 1.8 improves tax calculation accuracy for VAT merchants and eliminates potential rounding errors that can be confusing to buyers. Knowledge Base updates also provide more guidance on tax configurations and Canadian tax requirements, enabling you to more fully benefit from the native Magento tax engine. – See more at: http://www.magentocommerce.com/blog/comments/magento-community-edition-one-eight-now-available/#sthash.M1mhz8W8.dpuf

Functional Improvements
We made approximately 350 functional improvements that boost product quality in key areas, including the web store, shopping cart, admin order creation, import and export functionality, web API components, and payment methods. – See more at: http://www.magentocommerce.com/blog/comments/magento-community-edition-one-eight-now-available/#sthash.M1mhz8W8.dpuf

Performance Improvements
Magento Community Edition 1.8 enables a better shopping experience through faster page-loading in the checkout process and optimized cache adapters for single server systems, which minimize the number of cached pages that must be refreshed when product updates are made. The performance of the 1.8 administrator panel is also improved for those of you with large product or order databases. – See more at: http://www.magentocommerce.com/blog/comments/magento-community-edition-one-eight-now-available/#sthash.M1mhz8W8.dpuf

Security Enhancements
Through a number of enhancements, we also strengthened the security of Magento Community Edition. A full list of enhancements is available in our release notes. – See more at: http://www.magentocommerce.com/blog/comments/magento-community-edition-one-eight-now-available/#sthash.M1mhz8W8.dpuf