I was there from early in the beta. The thing that annoyed me, when I suggested that people should take a look at it was the decision that you had to make an account to see content. Most people visited the homepage and then said 'no'.
This is one of the main reasons i detest pinterest. Lately they have changed, so that you can see small amount of the content, before annoying you with the login screen.
Ditto for Quora. I see interesting content, but they annoy me so much with their accounts (even when I'm just reading) that I never spend much time there. Don-t worry, I will create an account when I have something to say, but until then just let me be.
The Gardened Wall model. Interesting gambit.
This did them in. If you run a community-oriented website you can't just promise me the moon and then tell me "Your name's not Dan, you're not coming in."