A letter or post: What's the difference?

Yesterday's story on the NUJ apologising to Suzanne Breen, the journalist who refused to hand over source material related to stories about the Real IRA, about a letter published in the Journalist made me think about the difference between readers' letters published in print and posts on the internet.
I used to edit a letters' page and found to my cost that it could often cause more problems than contentious news stories.
I think the reason is that you want controversial letters to spark debate and you are often not as rigorous in looking for potential libels in letters, because you see them as comment, as you are in news stories.
Yet, on the web you can see an avalanche of posts which are completely libelous. I know some sites are monitored but there is an anti-censorship, anything goes culture on the web.
Also posts are poorly presented. The Breen letter in the Journalist which the NUJ apologised over was a page lead, had an illustration and an attention grabbing headline.
The internet is more permanent than print yet I think a published letter is more likely to attract the attention of m'learned friends.