On wikis sometimes having a unique background picture will be important. It'll be on every page of the wiki for people to look at. If the wiki has a the same one as the other wiki it might look sort of bland. With unique pictures that are colorful but on topic will make new users think " That looks pretty cool." or something like that. The Complpedia is a wonderful example of this. Wordmarks and main sliders are another important thing. The Complipedia has all of these.