Late as am to the thread...

* Instant death mechanics have been here since 2.0 so it's not a new thing to deal with healing power creep (although healing power creep is kind of a problem, and yeah, I'd say the ridiculous spam in o4s/o8s do show that...)
* Instant death mechanics aren't what makes healing hard. Vuln stacks and healing debuffs do. If someone fails an instant death mechanic and dies... it's acknowledged as that person's fault. Reset and soldier on. Vuln stacks, etc tend be more "Oh well the healers can just heal more, it doesn't really matter if I fail that mechanic."

My preference is more for DPS down debuffs. That really drives home the message "I shouldn't do that" without needing to instant kill someone, and doesn't punish the healer for a non-healing mistake.