I haven't deeply gone into the details, but it looks like arenas have the following elements:
1. Multiplayer only, although you can solo it
2. You must load a saved game from the ./arenas directory
3. There is no protagonist. The game doesn't even end if all the characters die. However, there is no auto-resurrect.
4. I imagine for one to make some arenas, one has to make a saved game ("scenario") that can be loaded.
Maybe one day, someone can make a BG: Defense of the Ancients...
Edited by Ascension64, 05 March 2011 - 10:53 PM.