As part of the fluid layout upgrade, Wikia will be switching their visual editor to a new look that will be fully functional and with additional features some time in the future. It looks like the article you edit appears exactly the way you want it to look in the editor without having to use a preview button.
Rest assured that Source mode will not be going away and that those users who do use Source mode will not be affected by this change. But for those that do use Visual Editor, would you like to try out this new feature? (You can also try it out on Community Test Wiki if you want.)
Eventually, once the new Visual editor is fully developed, the older one will be deprecated. We can activate it on Nitrome Wiki now for those who want to give it a go, editing on this wiki.