If your website has a constant, steady flow of visitors and you then embark on a backlinking campaign to try to improve your ranking, wouldn't the sudden increase in backlinks look unnatural?
I mean, which came first the chicken or the egg? (the backlink or the visitor?) Surely it's the visitor.
Also would I be right in thinking that no-follow links are a lot more common than do-follow? If so then shouldn't we be keeping a very close eye on the % of NF compared to DF? And if google takes this into account, what do you think would be a safe %.