fuel prices - http://epetitions.direct.gov.uk/petitions/347 (small decrease implemented, plans for the future shared)
immigration - http://epetitions.direct.gov.uk/petitions/19658 (basic plans shared, recently implemented changes listed, already heard some people having visa issues, so something is happening)
summer riots - http://epetitions.direct.gov.uk/petitions/7337 (no direct actions, but response means pretty much "this is happening anyways", there's some more information about what the local authorities can do)
Hillsborough 1989 - http://epetitions.direct.gov.uk/petitions/2199 (accepted, disclosure will happen)
financial education - http://epetitions.direct.gov.uk/petitions/8903 (this one does look like a non-answer, but maybe they're really doing the reviews now)
private pensions - http://epetitions.direct.gov.uk/petitions/1535 (no response yet, only crossed the line recently)
While I'm not crazy about all the answers, there was some effect (or at least an explanation or a commitment to the original plan). They're quite far from "a more efficient way of shutting people up" the way it's presented in the comments here.