Founded by Jonah Peretti, who can also boast a founding credit at the Huffington Post, the site has mastered the list, the social folksonomy ("OMG", "LoL", "Fail" etc), and some sort of balance between the serious and the trivial, the instant and the long form. It now attracts 40 million unique users a month.
Part of its success is down to what Peretti once described as the "mullet strategy" which dates back to his HuffPo days. To quote from Eric Alterman's 2008 New Yorker piece on the decline of the American newspaper:
The Huffington Post’s editorial processes are based on what Peretti has named the “mullet strategy.” (“Business up front, party in the back” is how his trend-spotting site BuzzFeed glosses it.) “User-generated content is all the rage, but most of it totally sucks,” Peretti says. The mullet strategy invites users to “argue and vent on the secondary pages, but professional editors keep the front page looking sharp. The mullet strategy is here to stay, because the best way for Web companies to increase traffic is to let users have control, but the best way to sell advertising is a slick, pretty front page where corporate sponsors can admire their brands.”
In reality, most content sites have some form of mullet strategy -- the stuff they want to show off* is on the homepage while the stuff that generates the bulk of the traffic is elsewhere, often several levels down.
Given search and social media are the main drivers of traffic this is hardly surprising: more often than not a reader will visit a site without ever seeing the homepage either on the way in or on the way out. This fact alone should inform how we design websites, concentrating more on the signals and signposts provided on an article page than on the look and feel of the "front page".
That is one of the many things UK journalism can learn from Buzzfeed, whatever misgivings we may have about its approach; its use of advertorials, its recycling of news from elsewhere etc.
By the way, there are more lessons on viral media from Peretti's presentation here.
*This, as others have pointed out, may be where the mullet analogy breaks down given those who sport one are more than happy to show off what's at the back.