For example: Advantages and disadvantages of REE 1.8.7 versus Ruby MRI 1.9.2 with the local directory search path differences. Specifically since the most common solution is to pass underscores File underscores to patch around the problem for most projects or gems? REE does offer faster multithreading and there are other API differences, but I am looking for more significant differences.
Community Wiki shouldn't be seen as a way of ignoring the rules on what can and can not be posted, instead it's an indication that the question or an answer is better owned by the community rather than by individuals. They should still basically sit within the guidelines on the FAQ regarding topic, answerability and so on.
In terms of making something community wiki, questions automatically become community wiki when they pass a certain activity threshold and generally can't be easily changed and can't be specified as such in advance.
I suspect it may be possible for moderators but it's a privilege that hi rep users ever get so setting up questions and asking people to change them isn't something that would be encouraged as there is such a limited pool of people who can do it.