The Magento project is owned by eBay. As you can imagine, there are considerable resources allocated to Magento for the Community Edition (CE) as well as their paid Enterprise Edition (EE).
Open Source Driven
Magento Community Edition is free and open-source. By using open-source you are helping 1000's of developers around the world that are involved indirectly and directly associated with Magento and all of the associated extension developers.
Responsive Default Theme
Magento 1.9.1 came out with a new responsive default theme. This helps speed customization of a new theme (always duplicate first) and enhances paid themes.
1000's of Extensions
Magento features 1000's of available extensions to customize and modify Magento for a variety of ERP systems and unique situations. You'll probably never need to modify the code.