Is it? Maybe not just as a hatemail filter, but someone who actually collates responses, tries to get the reason for criticism without getting into arguments, and tries to extract actionable ideas from overall internet response could be an interesting offer. Sounds almost like it could be uservoice.com's premium offer.
I agree - doesn't scale, but it's far from a bad idea. I've done it for someone who takes reviews very personally. I abstracted the useful criticism and didn't make mention of the petty stuff.
At least it subscribes to the "Do things that don't scale" mantra. Can you imagine trying to run that sort of operation but for the sake of argument instead of HN comments, YouTube comments.
On that note I'm surprised nobody's using the YouTube API to do some Akismet-like auto-filtering already.
[For StupidFilter, w]e've gathered a fairly large (225K+ comments) database of comments, primarily from Youtube, that ever-inspiring font of stupidity. We've implemented a web-based comment ranking system to seed our stupidity corpus and that's proceeding nicely.
* Not necessarily true, who filters for the filterers?