Each pixel shows the number of nodes in its corresponding area¹. But this year every point that has data in it is shown (i.e. there is at least one node at that location - last year only locations with more than 1000 nodes were included). Also, the slippy map has two more zoom levels which reveal even more impressive details like on this crop of the central Alps:
Here is a low-zoom image of the whole planet:
¹ Yes, this is Mercator map tile area, not actual on-the-ground area. Keep this in mind when comparing regions at different latitudes!
PS: The visualizations are based on a planet file I downloaded one or two weeks ago. It was processed using some custom scripts based on node-osmium, the graphics were made with gnuplot (just like last years') and finally the map tiles for the slippy map were cut using imagemagick. I could probably explain the individual steps in a separate blog post, if anyone was interested - let me know!