It seems that editing a question will push the question back on top of the "active" stack. Is there a minimum amount of editing required to push it on top, or will people be able to abuse the system by just adding a comma, a space, etc, to have their question always visible?
While there is a minimum edit size, it's pretty small, and it does always push the post back to the top of the list. However, there is already a partial safe-guard against abuse here. See What are "Community Wiki" posts?
So if you edit your post 10 times (not including a short grace period right after posting), it becomes Community Wiki and you no longer receive reputation. This is a disincentive for editing too much.