The big change in Ubuntu 11.04 is the move away from the default GNOME experience to now utilizing Canonical's Unity desktop interface. Within Canonical's world there are also improvements to the Ubuntu Software Center for rating and review support along with other usability improvements.
The Ubuntu 11.04 ARM port also features the new Unity 2D interface.
Other changes include a headless edition of Ubuntu 11.04 ARM, Ubuntu 11.04 Server for Amazon's EC2 platform
Package upgrades include Mozilla Firefox 4.0, LibreOffice 3.3.2, Banshee 1.9.5, Xfce 4.8, and KDE SC 4.6.1. There's also Phoronix Test Suite 3.0-Iveland with OpenBenchmarking.org support in the repository now to replace PTS2. Under the hood is X.Org Server 1.10, Mesa 7.10.1, GCC 4.5, Python 2.7, and the Linux 2.6.38 kernel.
The release announcement by Kate Stewart can be found on the Ubuntu mailing list with download links to the various beta flavors of Ubuntu 11.04 Natty. With there being no release candidate for Natty Narwhal, a second beta release is planned for the 14th of April while the final release is set for the 28th of April.