Because content is king 100% per. site ranking in Google Top page, this is depends on unique contents is very main part of SEO
I think yahoo answer are reasonable places to get some link! plus you can get traffics by helphing other ppls! comment at related niche, are also helphing sometime! just don't over board! cause it can hurt your site!
In my opinion there are Five Essential Parts of SEO Technical. Content / Creative. Tactical. Social. Reporting. http://www.gadgetar.com/