Google have updated their algorithm (software) a lot over the past few years and it's becoming really intelligent. One major update was that you can now damage your site. In the past, you could do all the SEO you wanted, the bad stuff wouldn't help and the good stuff would, which meant people commenting on every blog, forum etc and hoping google picked up a few of them as good links, now this has changed, bad links will harm you, and good links will help you.
Commenting isn't bad in Google's eyes, as long as it's done right, if you follow Matt Cutt's (Google's head of search spam) advice, he reccomends when commenting on blogs, you comment as your name, and leave a link behind, but commenting as "cheap guitars" will probably do you more harm than good.