A fantastic look at the why there is such a difference in readability between the labels on Google Maps versus the other major mapping web services. My favorite part is the trick that Google uses to make major cities stand out:
It seems that by removing cities immediately outside of major metro areas, Google is actually making these metro areas stand out even more on the maps. This, in turn, makes it easier for many users to navigate the maps, because they’re often using major cities as reference points when browsing. In any event, the removal of cities immediately outside of large metro areas also makes the maps appear less cluttered.