Just saw an interesting interview that Andrew Warner of Mixergy.com did with James Hong, founder of hotornot.com.
http://mixergy.com/the-woman-who-paid-me-1-million-in-affiliate-revenue-teaches-you-about-affiliate-programs-with-lisa-riolo/ 

Hot or Not is a dating site that started in 2000.
James says Hot or Not was pretty 'edgy' at first, with many people wondering 'is it porn'? 
So for a while, it was too edgy for the big public companies to copy. 
Eventually, AOL came out with 'Rate a Buddy'.
But by this time, Hot or Not was such a common concept that the world had become 'de-sensitized' to it.

This reminded me of a great blog post that talked about why Blink.com (a precursor to del.icio.us) failed, despite securing many millions in funding. http://www.aripaparo.com/archive/001456.html

One of the reasons was that back then in '99, Blink had a public catalog of tags, and people were sensitive about what information they put on the web for all to see. 
But a couple of years later, the "N-sync/spears/Pokemon/Potter generation had come of age online and readily adopted privacy-free collaboration because they'd learned that everyone thinks alike, so there's nothing to hide".  
Enter deli.cio.us.
 
So we became de-sensitized to privacy concerns, and we became de-sensitized to the 'edginess' of dating sites.
Sounds a lot like how we've become de-sensitized to violence and language in films and television over the decades, going back to Clark Gable's famous line in Gone with the Wind that mentioned the word 'damn' in a Hollywood movie for the first time. 

The other interesting thing about James Hong's interview is that he said they thought Hot or Not's competitors would fall into one of 2 classes......
they'd either be big guys like AOL or guys like them...startups that would be relying on the same growth methods........word of mouth and press.

So for the first 6 months, all he did was get as many articles written about Hot or Not as he could.
In this way, he effectively froze his copy-cat competitors out of press coverage (on the safe assumption that the press wouldn't be interested in writing the same story twice: writing about a copycat once they'd written about the original).  

So there's a couple of interesting lessons for startups here: 
1. if you've got something 'edgy', or something that can be MADE to be edgy, freeze out the big gorilla competitors by exploiting the window after your launch before the market becomes "de-sensitized" 
2. get as much press coverage as you can to freeze out your start-up, copycat competitors.