As far as the features go, this is what has been added since 8.10 —
- Linux kernel 2.6.28
- GNOME 2.26
- X.Org server 1.6
- New style for notifications and notification preferences
- Boot performance
- Ext4 filesystem support
- And other stuff
A complete list along with download links can be found here.
The most noticeable among these I think is the boot performance. Ubutnu 9.04 still uses ext3 as default root but provides the option to use ext4. I decided to go for ext4. I dont know whether it is ext4 or the boot performance improvements, but startup and shutdown are noticeably faster than 8.10.
The new style for notifications and notification preferences is also a good addition, but it proves to be more of annoyance in certain cases ( pidgin and empathy). Moreover the options for enabling and disabling this aren’t very well maintained. In pidgin you need to disable a plugin to get it to stop — not good for the new user. Basically, this system needs to be polished a bit more.