3 and 4 seem reasonable. 1 seems not so. Spam is dealt with pretty well, userpages have very low Google rankings, and external links already have nofollow tags so they don't add to page rank.
2 seems also problematic because there are conversation threads that start with highly downvoted comments but themselves contain useful remarks.
2 seems also problematic because there are conversation threads that start with highly downvoted comments but themselves contain useful remarks.
That in itself is problematic - if they were barred from Recent Comments, maybe they'd be moved out from under downvoted comments.
A brief line from this comment indicates that the author of the cryonics-critical comment quoted here was perhaps not the one that deleted it.
Was it deleted by a moderator?
Honestly, the decisive downvoting seemed to do the trick of hiding it from casual readers who don't want to see the long annoying rants. I don't think it was casting any doubt on the credibility of cryonics.
While it sounds like the author regrets posting it, I would think they should be allowed to delete it themselves.
Edit: Originally titled "Cryonics critical comment deleted?"