Justin O'Beirne takes a close look at the most popular online map sites to try and figure out just why Google Maps is more readable. It comes down to the finest of details: Google adds white outlines to city names that are just thick enough to conceal what is behind the text, has a finely-tuned contextual hierarchy of type sizes, and a carefully selected color scheme.
As an aside, it's intriguing how each service's maps artistically reflect their corporate operators' natures. Google's is perfectly organized and functional, devoid of embellishment. Microsoft Bing's is beautiful and overdesigned, with a subtle palette of lavender and teal. Yahoo's looks like someone vomited a spaghetti dinner in Carrot Top's hair.
Update: Wow, Tumblr has a bandwidth limit? Here's a cached version of the site if it's down for you.