The "More Stack Exchange Communities" search box is useful for finding specific Stack Exchange sites, when their URL doesn't necessarily match their content. For example, if one is looking for the Stack Exchange for TV shows (http://movies.stackexchange.com), it's nice that she can type "TV" into the search box and get the correct result.
It seems, however, that the search is limited to strictly text within the sites' names and descriptions. E.g. if one can't remember that the Stack Exchange for video games is called "Arqade", he may try to type the phrase "games" or "video games" or "videogames" into the search box. This will not return the result for "Arqade", since neither the site name or description contain these phrases (the title and description, as of this writing, are "Arqade - For passionate videogamers on all platforms").
A potential improvement would be adding a system of tags to each site within the Stack Exchange network. That way, the concerns of having a cool site name and a functional description are separated from the functionality of search elements.