Social networking integration
Lucid gets a nice social Networks Integration with “Social From The Start” initiative powered by open source project Gwibber.
Cloud Sync & Ubuntu One
Ubuntu One brings a commercial cloud synchronization service to the Ubuntu desktop. Major parts being mobile synchronization (iPhone, Android, others) and a New Music store (iTunes Alternative) whih wors with popular Ubuntu player: Rythmbox.
Apart from what is said, there are lots of bug fixes and even some nitty-gritty stuff like inbuilt support for iPhone Sync on Ubuntu.
Ubuntu 10.04 Lucid Lynx is available for Download at http://www.ubuntu.com/getubuntu/download.