Any Windows fans out there will be pleased to hear that ext4 includes an online defragmentation tool, e4defrag. However, this doesn't appear to be installed on Ubuntu 9.04 (or perhaps it resides under a different command-name; if you know the situation, post a comment below.)
There's a new boot progress graphic (i.e. usplash), which I can't say looks better or worse than previous efforts. Additionally, there's a new wallpaper that takes as a starting point the swirly lines seen in virtually every operating system wallpaper since 1998. The only different here is that the color scheme is orange and brown, in the usual Ubuntu style. My advice: install the gnome-backgrounds package, which includes a terrific sample of images.
Some of the community themes that were optional in 8.10 are now default (Dust, Dust Sand, New Wave), and this helps mitigate the fact that the whole Ubuntu desktop experience is starting to stagnate. It hasn't changed significantly for quite a few years now.
It's now possible to set transparency effects for the panels, provided the Compiz visual effects system is activated (it is by default). This is kinda cool, and brings Ubuntu into line with OS X Leopard, which introduced a similar feature.
The Log Off/Shutdown etc. items have been removed from the System menu, and now live on the fast user switcher icon at the top right of the desktop. This icon is turning into something of a status display and mode-switcher--not only does it now let you log out etc., and switch to a different user account, but it also shows your Pidgin status. Rather annoyingly, the fact it is now the only way to shutdown means that you can't get rid of it, unless you intend to use telinit each time at the prompt.
The last few releases Ubuntu have featured notification bubbles that pop-up near the system tray area at the top right of the screen. These inform the user about events that have happened, such as connecting to a wireless network.
With 9.04 the notification system has been visually overhauled so that the pop-up boxes have a smoked glass appearance. All notifications now appear as pop-up boxes in this style, including notebook screen brightness status displays, for example, or low battery warnings. If more than one notification appears at the same time, they stack-up beneath each other.
Unfortunately, the notifications don't work like those on other systems, whereby you can click on them to clear them. Put your mouse over the notifications on 9.04 and they turn semi-invisible, letting you click beneath them. They only go away when they want to, which seems to be after a couple of seconds by default. This is a little annoying.
Although it was skipped for the 8.10 release last year, despite being available (the omission was caused by problems with packaging, apparently), OpenOffice.org 3 has made it into the 9.04 release.
To be blunt, this is very hard to get excited about. There's a handful of cool new features, including inline commenting, which as an author I find useful. If you want to know more, see the OpenOffice.org Web site.
Synaptic now features a "Get Screenshot" button in each package description that will, as you might expect, download a thumbnail screenshot of the application (provided it's the type of application that it's feasible to have a screenshot of; system components don't have them, for example). Clicking the thumbnail will then download the full-resolution version. This is a nice feature.