I just read the recent survey results post (which was very insightful), and saw the comment about the test server. I think since it has so few people logging in, AND the development team has been incredibly responsive in 2014 with the speed of updating the game, it should be reserved for major system changes.

Examples would include: the new user interface, monstrosity, etc. Anything that is unlikely to be part of a small update or by adding/changing it would be a great departure from the game's previous state.