Not to navigate by car, by bike, by foot. Not to show advertiser bubbles, or utilities.
Just a clear communication of "The point in question is in this neighbourhood, at that point.".
For me, that's the beauty in MapBox's Streets style... it does that one thing really well. Retaining enough countryside detail to make location recognition very good, and it works brilliantly in a city. Yet at the same time it's neutral enough to look great no matter how bad the design of the web-site it appears on.
I definitely prefer non-US, and non-giant mapping providers, but there's a lot to be said for cartographers who design maps to be useful generically for location recognition rather than anything else. I strongly feel that this is what Google Maps got so right, and is also why MapBox will do very well.
More than the tech, design matters.