The great thing about it is, not only on GNU/Linux PC/Laptops... it can also be run in MS Windows, Mac OSX as well. And as mentioned above, the most "important" feature behind its popularity may well be because of the gorgeous look-n-feel it inherits... which can be foreseen :P since they've used Qt toolkit to build the GUI.
*. According to the developers themselves, it supports 270 different types video/audio file/containers! such as... AAC, AC3, ASF, AVI, DIVX, FLV, H.263, Matroska, MOV, MP3, MP4, MPEG, OGG, QT, RealMedia, VOB, Vorbis, WAV, WMA, WMV, XVID, etc.
*. Can play both encrypted/unencrypted DVDs as well.
*. Fully featured equalizer.
*. Adjust screen hue/brightness, etc.
*. Audio gain.
*. Take screenshots.
*. Adjust audio/video delays (very handy sometimes, for instance while watching a movie - if you hear the gun shot sound before the video and wondering how to fix it ....).
*. Lots of audio/video filters.
*. Easily change skins.
*. Online subtitle search... are just a few to mention.
How can I install it in Ubuntu 11.04 Natty Narwhal?
Not just for Ubuntu, but you can obtain builds for Debian, RPM based packages, MS Windows, etc from this UMPlayer download page. Once the downloading is complete, just double click on the file and follow the instructions on your screen. That's it!.