People using Umbraco to create and edit content will find it intuitive and familiar. While the built-in content tools allow you to seamlessly edit each page of the site you're working on, it's also possible to manage content directly from Word 2007, making it particularly accessible to people who haven't used content management systems in the past.
The features I like most are Umbraco's new site building tools. Boost is a tool used to create the basic framework of a Web site during the installation process. Into this framework you can plug in various modules -- which Umbraco calls Nitros -- that add functionality, such as menus, forms, and site maps. Umbraco has historically demonstrated a strong commitment to supporting Web standards, so further tweaking sites built with these tools should be fairly straightforward.
Another notable improvement in Umbraco 4 is broader database support; you can now use MySQL, MSSQL, and VistaDB, which certainly makes the platform a lot more flexible.
Umbraco 4 makes some pretty significant improvements across the board that I believe will win it a lot of new fans. While it's often overshadowed by DotNetNuke, Umbraco has emerged over the last few years as a solid choice for organizations seeking a .Net-based content management system. You can download Umbraco 4 from the CodePlex open source software repository to find out for yourself.