How important is that a backlink anchor text contain words from the website content? For example, what will be the effect if completly offsite exotic marketing phrases are used? It will be considered keyword spamming?
It's the principle behing google bombing - http://en.wikipedia.org/wiki/Google_bomb supposed to be harder to do now, but it still happens and still works.
Relevance is pretty important in my opinion. But since google keeps their secret rules a secret no really knows for sure. I try to the big three for me. 1) DoFollow 2) High PR - It doesn't have to be 5 or greater... I just stick with 2 and above 3) Relevant. I only comment on blog postings related to my linking site. I've personally had great success with this formula but everyone's opinion will differ. Hope that helps.