It's not things I want to be able to ignore, it's people.
As it is, if I want to apply a block for a particular individual across various applications, I have to create multiple ignore rules for RSS readers, social invites, IM etc. As an example, it was easy enough for me to create a Thunderbird filter for removing posts from particular bloggers to an aggregated RSS feed, but that filter doesn't keep my Inbox clean. I also sometimes read the RSS feed in HTML, so to protect myself from contamination, I had to duplicate the filter as a Greasemonkey script that removed all offending posts from the page.
Forget the burden of maintaining my social network across applications, I'm spending too much time maintaining my anti-social network. And as I age and become more irritable and less open to diverse opinions, I can only imagine that this will become more relevant to me.
I want a universal social blacklist. As a first step I want to block any attempted interaction with myself from colleagues, bloggers, & spammers. In stage 2 we'll move onto journalists & politicians.
There may even be instances in which I would want its contents made public (likely with myself anonymized). I know I'd be personally interested to know how many people have me on their blacklist (a theoretical possibility of course).
Let's call it inattention data.