It is more faster (both loading the app it self + web page loading times), more simplified (thanks to Chrome/Chromium I guess) and is already a heavily used browser on mobile phones as well.
Here are the main features...
*. Thumbnail previews of Tabs - Whenever you bring your mouse over a tab, Opera 11 automatically creates a thumbnail preview, which is pretty cool!.
*. Easily skin-able - Currently there are hundreds of cool looking skins available for Opera.
*. Built in RSS feed reader.
*. Widgets -
With Opera, you can make use of a lot of beautiful looking widgets which runs even when you close your browser.
*. Search - Not only a built in search engine field, but you can search your history, bookmarks with ease as well.
*. Lock tabs - Unlike with Chrome or Firefox, Opera can be set to lock your tabs which prevents accidental closings.
*. Easily zoom in-out web pages.
*. As said, it is getting seriously faster than the predecessors.
So, if you want to install Opera 11 in Ubuntu, then get it from here and click download opera button (make sure you've chosen Ubuntu in "choose package format" option). If you want daily updated beta releases then get it from here.
Then click on the .deb package once downloaded which should open the Ubuntu software center window (as below), then from that window, click "install".