And here's someone else's (equally lacking in cited references) opinion, stating that one of WordPress' advantages is its "huge community".

For an attempt at relevant comparisons, let's start with community gatherings.

For purposes of brevity, I'll ignore WordPress MeetUp gatherings, and just compare the number of Joomla Days events with the number of WordCamps. Joomla appears to have about 25 this year. WordPress appears to have about 55 this year.

Advantage: WordPress

Next, we'll look at community-contributed extensions.

Joomla appears to have just over 8,000 extensions (I cannot find a separate listing for themes/templates) contributed from its community. WordPress has over 16,000 Plugins, and almost 1,500 Themes contributed from its community.

Advantage: WordPress

Next, let's consider the official support forums.

Joomla's official support forum has about 2.4MM posts, in 575,000 topics posted by over 500,000 people. WordPress' official support forum has about 2.4MM posts, in 590,000 topics posted by over 2.7MM registered posters (according to Forum moderator Andrea R.).

Advantage: WordPress, based on # of registered users, though this could fairly be considered a push overall.

How about community-contributed documentation?

We could compare community-contributed documentation, but I cannot find a good metric to compare Joomla's documentation against WordPress' documentation.

Advantage: (undetermined)

What else is relevant?