Drupal has a great community now with lots of volunteers who have been helpful to me personally on numerous occasions and who really value community-building. Having said that, I think there are also lots of ways that we can improve. I used to be fairly involved in the Wikipedia community, and although they're not perfect either, I think it would be worthwhile to study some of the ways that they welcome new people and encourage participation. I'll just point out a few things:
(1) The Wikipedia home page tries to be very welcoming especially to new contributors. It prominently displays the slogan, "Welcome to Wikipedia: the free encyclopedia that anyone can edit." Clicking on "anyone can edit" takes people quickly to an introduction page which is written with newbies in mind, gives basic information about how to edit, and encourages them by saying, "Don't be afraid to edit. Anyone can edit almost any page ... Be bold! Why not try out our editor right now?" Not only does it repeatedly and emphatically beg people to participate, it provides a way for people to get started that requires very little training or technical knowledge. People don't even need to know much about the topic of an article in order to edit it, and many people actually provide useful contributions by simply making minor improvements to spelling, punctuation or wording.
Drupal.org isn't terrible about welcoming new people and introducing them to things that they can do, but it places the bar to entry somewhat higher. Instead of "Welcome...anyone can edit," its slogan is, "Come for the software, stay for the community." There is nothing right away that says, "you can participate." Clicking through to the community page tells you how to find events, IRC chat, forums and mailing lists, but you have to click through to still another page to get to the "Getting involved" page. Most of the ways to get involved, aside from posting to the forums, require a significant degree of prior Drupal knowledge.
To some degree these differences exist because Drupal is a software project, which seems to inherently require greater technical knowledge than WIkipedia. With some ingenuity, however, maybe Drupal could create more participation opportunities for new people.
(2) Wikipedia has a highly differentiated community of users who have chosen or have been assigned roles related to maintaining the community. Roles on Wikipedia include: administrator, bureaucrat, steward, reviewer, rollbacker, file mover and account creator, which confer permissions to block other users, roll back edits, protect pages, assign roles, etc. In addition to these formal roles which are have specific permissions inscribed in the software, Wikipedia users have also organized themselves into numerous teams with specific tasks, such as the Wikipedia Welcoming Committee or the Counter-Vandalism Unit. "WikiProjects" organize groups of editors that want to work together as a team to improve some section of the encyclopedia, with tasks such as stub sorting, scientific peer review, citation cleanup, copy editing, translation, usability, etc. It also has a team whose specific mission is to award "barnstars" -- icons that are placed on user pages to praise them for their volunteering. Joining most of these teams is easy and requires simply adding your username to the team's project page. To further encourage volunteering, Wikipedia's community portal lists dozens of open tasks, fix-up projects, and tasks requiring special skills.
Again, I can't say that Drupal is bad at community building and suggesting tasks for would-be volunteers, but it doesn't have as many options for people to organize themselves into teams, and often people have to undergo a challenging approval process before they are allowed to participate. A friend of mine waited more than six months before receiving approval as a module contributor. When I attempted to advocate on his behalf by posting comments about his situation in IRC, I was told that that this sort of delay was increasingly common because there are not enough people volunteering to review new contributor applications. They thought this shortage of volunteers stems from the fact that "everyone wants to write new modules," but there aren't very many people who are willing to take the time to serve in a reviewer capacity. I think it is fair to say that Drupal also has a shortage of volunteers working on improving documentation. Is this because those tasks are not very important? Is it because most people just don't want to do them? Or could the Drupal community do more to announce the need for specific volunteer tasks and then reward volunteers with recognition?
Drupal.org has its own systems for organizing subcommunities -- forums, groups.drupal.org, IRC, meetups, DrupalCons, etc. However, I don't think it puts as much focus on organizing that empowers participation. Maybe this is because Drupal serves a different purpose than Wikipedia, or maybe it's because Wikipedia's user base is larger.
In one respect, however, Drupal actually does a much better job of organizing volunteers than Wikipedia does -- specifically, software development. The contrib modules and themes sections on Drupal.org are hives of activity -- people submitting tickets, patches, new modules. Wikipedia has its own system for maintaining the core Mediawiki software and third-party add-ons (called "extensions" rather than "modules"). However, it doesn't have anywhere near the same level of organization with regard to bug tracking, revisioning and version releases, and as a result software improvements have not been as rapid.