Quality: if somebody likes your site they link to it.
Reputation: if you are well-known in your community or industry, you're more likely to get linked.
Cooperation: if I link to you, and you link to me, then we've both increased our PageRank.
Wikis provides a web spider with a dizzying web of interlinked pages. The average Wiki page is far more linked-to than the average web page, which inflates its "goodness" in Google's eyes. For example, in early 2004:
a search for "Iraq WMD Location" brought up this wiki's page by that name, mostly on the strength of intra-wiki links
a search for "aftermarket pipes" brought up not motorcycle shops, but an obscure and little-read web log by that name
a search for WikiZen and programmer LayneThomas came up first on Google ahead of the arguably more well-known Layne Thomas Staley (from the popular U.S. band Alice In Chains).
As with any system, the rules create unintended consequences. The system also rewards:
Money: you can simply buy several domains and link them all to each other. In a small country, or in a small industry, this is a common and very effective tactic.
GoogleBombing: by creating enough links to their pages, people can boost awareness of their message out of proportion to its real presence on the Web. This has been used in both harmless ways (propagating a nonsense phrase to see how high the PageRank can get) as well as less honorable ones (promoting self-published books on Amazon.com). Theoretically, it could also be used by people with an obscure agenda to make their opinions seem more mainstream than they really are.
Spamming: as a special case of GoogleBombing, spammers have discovered that editable wikis and blog comment areas are easy ways to promote the page rank of their sites (as reported by SlashDot at http://slashdot.org/articles/04/06/07/1623244.shtml?tid=111&tid=126&tid=95). The self-healing nature of popular wikis helps defend them (spam doesn't tend to last long with a large number of motivated volunteers), but blog comments aren't as resilient; many bloggers have turned off comments to avoid this.