Some of the packaging highlights to Ubuntu 11.04 include LibreOffice 3.3.2, the Linux 188.8.131.52 kernel, Firefox 4.0, Banshee 2.0, Mesa 7.10, GNOME 3.0 packages, X.Org Server 1.10, and GCC 4.5.
There's also now Ubuntu ARM Netbook ISOs that use the Unity 2D desktop by default, headless ARM ISOs, various improvements to the Ubuntu Server builds, and Kubuntu 11.04 is running with KDE SC 4.6.2. Xubuntu 11.04 is based upon Xfce 4.8.
The release announcement for Ubuntu 11.04 can be found on the mailing list. Many more details can be found in the Ubuntu 11.04 release notes.
As mentioned in the release notes, the nasty power regressions are not fixed in the stock kernel so some systems will be consuming 10~30% more power compared to previous releases.