FWIW, Wikipedia's Visual Editor has been in public beta since December, and it's arguably a much harder job because of the baroque complexity of MediaWiki's markup.
http://en.wikipedia.org/wiki/Wikipedia:VisualEditor
Not that this takes anything away from MapBox's achievement, and OSM were foresighted in choosing a sanely structured data model that allows things like this.
The Javascript libraries they're using are interesting. D3 is a big part of it, but equally notable is that there's no conventional slippy map library like OpenLayers or Leaflet. Instead they're doing something simple with d3.geo.tile, a relatively new component.
Particular attention is due to MapBox's OAuth implementation ohauth, purely client-side (no server proxy): http://mapbox.com/osmdev/2013/01/15/oauth-in-javascript/ They also use RTree (for spatial indexing), Lo-Dash (an Underscore alternative), and a few extra bits like JXON for XML, the async library Queue, and Bootstrap tooltips.