I missed the best known feature of Ubuntu 9.04 in my roundup, the new notifications. There’s lots of information around about this, see Mark Shuttleworth’s announcement, LWN coverage of development debate and Glyph Lefkowitz’s criticisms for plenty of information (Matthew Paul Thomas comments at Glyph’s place extensively).
I just want to pass on one tip, which is how to turn off notification of new messages in the Pidgin instant messaging client, because good grief, is that annoying. My typical use of IM is actually somewhat asynchronous, as in, I will flip over to it and check for messages periodically, chat a bit, do some work, and back. The notification workflow assumes I want to know about every message as soon as it arrives, which I most emphatically do not.
Other people of my opinion can configure Pidgin’s notifications by going to ‘Plugins’ and then looking for ‘Libnotify Popups’, which you can either configure to be less talkative, or disable entirely. In an ideal world that would be slightly more discoverable: I expected the configuration to be associated with the notifications, not with Pidgin itself, and certainly not with a plugin in particular.