It's certainly better than anything I'd thought of, but I don't really think of WtP as a user-based site in the same way that Digg is/was. People don't, I suspect, sign up for an account and then regularly visit and choose which petitions to sign. Rather, someone starts a facebook campaign, people go to the site, sign up, vote, then forget about it. I'm guessing regular emails from WtP inviting someone who once voted to ban abortion to come back and vote on a random issue probably wouldn't work out very well.

The site could, instead, be turned into a more community-oriented place, with discussions on the proposed petitions and a sense of being part of the in-crowd when one you've talked about gets answered.

In fact, this is starting to sound an awful lot like Wikipedia, with the same kinds of drives and motivations. Create a set of guidelines for community interaction and petition content, then let "contributors" monitor proposed petitions, cull the obvious crap, improve the writing and cases cited, then publish them to the "new" page. You could have a couple of White House interns be mods and settle any disputes between members.

I'm just brainstorming, here. In reality, it seems like an idea that's nice in principle but unworthy and unworkable in practice.