Q:

What elements make a game machinima-friendly? 

What types of things should a game have in order to make it machinima-friendly?  I imagine that a spectator mode helps, but are there other important elements?  Perhaps support for mods? 
Are there any example of games that are known for being especially easy to make movies with? 

A:

RuneScape has put forth some effort into supporting the machinima community.  They introduced an in-game item, the "Orb of Oculus".  A player holding the Orb of Oculus is able to freely move and rotate the game camera (to an extent, of course); this essentially makes that player into a "cameraman", and they can record other players from interesting angles (since the camera in normal operation is always hovering above and centered on the player's character). 
Here is the manual page on the Orb of Oculus (and other fun items for provoking emergent gameplay), and here's the news post which links to the blog post about it.  You can, of course, see the results by searching on YouTube for RuneScape machinima videos. 
It has also been used to look at the other sides of impassable boundaries and find hidden unreachable areas, and to discover how two areas which might appear to be in two completely different places in the game world, are actually just off screen of each other.