If you look back at the early days of this site (via some of the archive threads) you'll see how while comment moderation was always a slipshod affair, there was at least an effort to keep things civil and delete the more offensive comments. It now looks like a complete free for all. As said elsewhere, I hope that it's a small number of obsessive inadequates who feel emboldened, rather than an actual groundswell of opinion.
I have a suspicion that the Mail comments retained a veneer of civility and sanity during the period the Telegraph - and Telegraph blogs in particular - had below the line comments. Unmoderated and unrestrained racism, anti-semitism, misogyny and Islamophobia. When the Telegraph shut down comments and their bloggers all went to Bretibart, the commenting nutters had to go somewhere and just overwhelmed the Mail site.