This site is incredibly useful but the concept can be improved by adding three features
Comments
By allowing individuals to leave a comment below it creates a forum for open discussion which can lead to improved petitions that become more inclusive
Similar Petitions
There should be a system where similar petitions are displayed to the right of the current petition you're browsing.
Up-voting
Every comment should have two adjacent buttons, either the ability to up-vote or down-vote a comment. By implementing this system the most informative and inclusive comments will be seen directly below a petition while comments that benefit a select few are placed towards the very bottom of the page.
See reddit.com as a model for how their site brings relevant content to the user