There has been an immense increase in forum postings, Question/answers and blog comments from SEO point of view. No doubt, there are people that do all these techniques for genuine reason, do take participation in forums to discuss, share their opinion and gather good piece of knowledge with other and do comment on blogs to help other people and seek information from these tremendous ways.
Unlike other techniques, Forum posts has the ability of giving you the chance to have a high keyword density for your webpages without having to have duplicate content. You participate in forums, answers, share your opinions leaving behind the link of source i.e. Keyword and the URL. There is one criteria to differentiate the results whether it is going effect positive or negative. Keyword density is one of the criteria to get a higher ranking when Google robots will crawl it.
And, you will be shocked to know that this is the only one criteria where most of the sites result in penalized for keyword stuffing. The discussions on forums include your keywords but few things to keep in mind is that you need to optimize the keyword rather than stuffing it. Similarly, it goes for blog commenting, making sure that you are posting relevant comment, that help web users and do not intend like you have made comment for leaving your link. So, I can conclude saying that genuinely working on such platforms can not give bad ranking but can help in boosting rank if authors of forums and blogs like your comment and give a link site.