The problem is that the interface skin used by WMF projects is also the default skin in the MediaWiki software. This means that there are thousands of wikis out there that look exactly like Wikipedia et al. How is anyone (even an experienced user) supposed to realise that WikiTravel [2] is not a WMF project when it looks exactly like one? I think that the strongest thing we could do to reinforce the brand is to create a new skin that is used by WMF projects _and_only_ WMF projects, which is copyrighted/trademarked (if that is possible) and which is not included as part of the MediaWiki code.
One of the most common objections to Wikia's new skins is that they don't look like Wikipedia. To many people wiki=wikipedia and they don't understand why a wiki would not look like that. It's not only an issue of design, but of branding. People associate the monobook skin with a professional, neutral site and they want their wiki to have that same branding.
Angela