The flipside is that the automatic warnings reduces the problems with 2. Once the warning occurs, the discussion can simply be moved to, well, the discussion area. (I'm not begging for new LW functionality, I just mean that when someone sees that waring, they can instead just start something in the discussion area and maybe leave a link in the thread to the new discussion)
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?"