EnSight 10 is just packed with improvements.
The top 10 benefits of EnSight 10 are: (you can argue the order)
1) New user interface
2) Faster, more practical Volume Rendering
3) Further improved CEIshell for HPC sites
4) Improved plotting
5) More data readers
6) Drag and Drop
7) Improved Calculator
8) Internationalization - menus and documentation (so far Japanese and Chinese)
9) PythonExchange python scripting community
10) Enhanced customization
Mac users, this mac_ensight_10 (pdf) mentions other items specific to the Mac.
This wiki provides guidance on where things have moved, how they work compared to EnSight 9.