Blog comment links and forum signature links are not
high quality links.
Neither are the vast majority of guest blog post links.
high quality links (the kind of links that Google is talking about when they say "high quality links") are editorial links given by other webmasters because they're so impressed with the usefulness and quality of what you offer they want to share it with their site visitors.
These kinds of links are very, very hard to get. That's the point.
High quality links are supposed
to be hard to get.
Any link you can create yourself, any link that is quick and/or easy to obtain (i.e. all the kinds of links mentioned in the previous post
) are not
considered "high quality" by Google. They may work... somewhat... for now... sometimes... maybe, but for the love of sanity will people please
stop recommending them as "high quality links"? That is one thing they never were and never will be.