I have to agree with Frosty here. The page that is linked in the summary clearly identifies the problem in the section entitled Rationale; "The decline in new contributor growth is the single most serious challenge facing the Wikimedia movement in the year 2011." Unfortunately they come to the wrong conclusion as to how to address the issue with the very next sentence; "Removing the avoidable technical impediments associated with Wikimedia's editing interface is a necessary pre-condition for increasing the number of Wikimedia contributors."
Quite frankly, it's obvious that the "technical impediments" of the editing interface are not to blame or else there would not currently be 4,099,684 pages of content (which excludes an additional 24,635,011 "other" pages - source: http://en.wikipedia.org/wiki/Special:Statistics
) as I type this. No, as Frosty P. states the problem is with the drama that comes with attempting to edit or create articles on Wikipedia. Rampant deletionism (which wasn't a thing before Wikipedia, hah!) abounds and new users are driven away in frustration. In short, they need to work on getting their current volunteers to operate in a more welcoming manner.
No doubt a majority of the problem is caused by a minority of the editors, but like everything else the vocal minority will out-influence a silent majority. This is the problem.