Just as Pinterest is set to pass Tumblr in traffic and reach according to Alexa , much of the attention that it received in the past 4 months  is starting to wane after shallow growth in March. Just as Twitter and Tumblr before it, Pinterest is due for another boost in traffic and users in the next couple of months, but it's not the type that current users want.
The spammers have Pinterest in their sights. It was inevitable; any site that sends the kind of traffic and receives the level of exposure that Pinterest has been blessed with since January was bound to get hit by spammers. The difference here is that the damage can be greater if they do not change a few things.
Adjust the Popular Algorithm
It didn't take much scrolling on the popular page of Pinterest  to find two examples of challenges that the site will face as the spammers and marketers start to figure out the promotional algorithm. Reposts will always happen in social media and that's not necessarily a bad thing. Those who believe that content only gets one chance at exposure spend too much time on these sites.
The problem with reposts reaching the popular page is that clever traffic spammers can find something that works well enough to hit the popular page once and recycle it later. There will be enough posting and reposting of recycled material without the enticement of getting thousands of visitors to a spam site by plugging in the same foldover dress from last week.
The second example above deals with gaming the system. While it's definitely possible that the layout of the apartment pictured above is clever enough to get some attention, it's not the type of content that can reach the popular page without some manufactured likes and repins.
To fight this, Pinterest will need to first establish an image search component into their promotional algorithm that raises the threshold for repetitive content. It isn't that great pictures do not deserve to reach the popular page more than once, but hitting the second time should be harder than hitting the first time, and the third time should be even harder. By the time the same image has hit the popular section 4 or 5 times, it should be almost impossible for it to hit again for a long time.
As far as those who are able to game the system, they will need to implement a domain-based algorithm trigger that will force low-quality domains or free posting sites such as Blogspot to receive higher likes and repins before hitting the popular page.
Another option, if they do not want to limit domains the way that sites like Digg and Reddit do, would be to establish credibility measures for the people repinning and liking content. Someone who has been on the site for 10 days who does nothing but repin content from the same site over and over again should have less power towards promotion than someone who represents actual users - pinning regularly, establishing trust, and truly "using" the site the way it should. These types of measures are already present in Google+, Tumblr, and Twitter, and implementing them would not be terribly difficult.
Create a Link Filter
It's a miracle that the nefarious types who normally spread malware haven't latched onto Pinterest in full force yet. The site is completely vulnerable as they do not have the type of link-filter that Facebook and Twitter use to prevent users from clicking on links that will take them to bad sites.
It won't last long. Some minor cases  are already starting to creep up.
Before the site hits the radar of the bad guys on the internet, they must put in a link filtration system. Bad links are not too hard to stop if they are explored after being posted. It's only a matter of time before the first big virus starts spreading on Pinterest, an event that the site cannot afford at this early stage in its development.
Encourage Branding (or avoid it altogether)
If you look at the brand names of the top 5 auto manufacturers, Toyota, Chevrolet, Ford, and Chrysler are all taken by individuals. Honda  is the only one who has a board and while they've done a clever pin or two, they have only 400 followers and haven't posted in 2 weeks.
At some point Pinterest will need to start contacting individuals who are squatting on potential brand names and put placeholders there for the day that is (hopefully) around the corner where they become a necessary component of social media communication for businesses. They seem to have blocked off some names (pinterest.com/google, for example) but a quick type-in of prominent companies will reveal a lady named Apple Ratana , a Zappos page  with no shoes, and not a whole lot going on besides a lady's profile picture at Harley Davidson .
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Pinterest has made some changes in the last month that addressed some of their issues, including porn, self-harm pictures, and copyright. They're not out of the woods in those areas, but they'll need to hit some of their other potential challenges before the spammer/marketer/malware problems start popping up more often.
They will. You can pin a board on that bet.