Player Experience Panel, Phillips at GDC 2010.
(slides, mostly about FPS games)
Each dot represents the average number of days taken to complete a particular achievement. DLC achievements are farther because DLC were released later (not because they are harder or take more time to complete).
Box gameplay peaks when new DLC is released: new content does increase the number of people playing. However, later DLC packs do not peak as much as the first DLC pack.