For one, the new desktop notification system has landed for Ubuntu 9.04, which looks nice if you are running on a composited desktop. While Canonical has invested significant resources into this new notification system, not all are happy with the current implementation as some have voiced their concerns.
There is a new log-in screen for Ubuntu 9.04, which is mostly black with only a bit of orange. However, the completely new desktop theme will not be here until Ubuntu 9.10 (the Karmic Koala).
The Ubiquity installer for Ubuntu 9.04 has also picked up a new world map. Though also changed in the Ubiquity installer is the home encryption support, which has been removed unless you pre-seed the option to Ubiquity. This option has been temporarily dropped since there is no encrypted SWAP support, which reduces the security benefit of an encrypted home directory.
Oh, there is also a feature in Ubiquity right now that may make you think Ubuntu can multiply your hard drive storage capacity by many times, but it is in fact a bug.
Ubuntu 9.04 Alpha 5 should be out tomorrow (or Friday if there are any last minute delays) while the final alpha will be out in mid-March. The final release of Ubuntu 9.04 is scheduled for the 23rd of April.