Post-secret differs in a few important respects I think.
1) Its curated. There is someone to separate out the drivel. And because its curated and people know that, they're less likely to send in complete crap.
2) It relies on physical media. It takes some level of effort to make a card and send it in not to mention a small but non-zero cost. It also means that its not instantaneous like most things online.
3) There is no real feedback. There is no direct comment system, so theres no way to feed the trolls.
Hi, I'm one of the founders. Secret also has some important aspects worth discussing:
1) It's curated. By your friends. The posts you see are things that your friends are sharing.
2) Conversations are all within your circle of friends. As a result, we don't see much real trolling at all. The communities that are forming have been remarkably interesting and kind.
3) The penny arcade example is also not entirely applicable here. Again, because it's within your circle of friends. The equation is different.
You're probably noticing a recurring theme: We believe that by scoping anonymous sharing to your friends, the content will be much more positive and helpful. So far, this has remained true, and we're inspired to see the good come out in people when they're given the privilege of anonymity.