In this recent video, Matt Cutts (Search Quality for Google) discusses the potential risks of allowing users to post comments with link spam on your blog. You have control over the content on your website, and if Google catches you linking to spammy websites, they’ll likely consider your website spammy as well.

He also touches on the reason Google typically doesn’t penalize your website for spammy in-bounds links. If Google penalized websites for any type of in-bound link(s), then competitors would be building the worst links possible to each other attempting to remove competition from the search results.

There are still plenty of blogs that do not add the rel=”nofollow” attribute to comment links. Why would a blog owner want to do this?

A dofollow blog comment link is a simple reward to users that add valuable comments and feedback. The blog owner gets valuable user-generated comment and the visitor who leaves a good comment gets a dofollow link. Most blogs that allow dofollow comment links strictly moderate those comments and links, so you’re not likely to get away with posting a link unless your comment contributes and adds value to the blog post.

Another reason a blog might allow dofollow comments is if the blog has been around for a long time and the owner doesn’t update the platform often. Believe it or not, there are thousands of blogs that have not been updated since the rel=’nofollow’ attribute was invented. Many of the blogs are still used and new posts are added regularly. Many of these blogs are moderated closely or filled with blog comment spam.

If you’re trying to build your backlink profile I would stay away from blog posts with other spammy comments. You’re not likely to get penalized from posting links next to spammy links, but Google often identifies these spam riddled pages and filters them from their link graph.

In most cases the effort of building your backlink profile by posting blog comment links is not worth the effort as these sites normally contain so many comment links that the PageRank value is divided too much to pass substantial value. Blog comment links with the rel=’nofollow’ attribute will not pass any value to your website.

Since backlinks have become a known commodity, many linking scams and services continue to pop up all over the web. Don’t be persuaded in spending the money for these services. Not only will you not improve your ranking, you may be putting your websites at great risk of being dropped from search engine indexes.

Yes, there are such things as bad inbound links.

Here is a quick list of some of the types of links you DON’T want pointing at your website. This list is not to be confused with links that simply to not pass value such as links with the rel=”nofollow” attribute.

1. Don’t get links from sites that blatantly sell text links.
2. Don’t buy into link farms. You may have run across a deal like, get 1000 links for $29.95 per month. Stay away from these programs.
3. Stay away from popular link brokers like text-link-ads.com or textlinkbrokers.com. These brokers will sell you text links with the direct intent of influencing Google’s search results. This is a violation of Google’s Terms of Service.
4. You will find users on popular webmaster forums offering to sell high pagerank links. Many of these sites are created by buying expired domains with high Google PageRank values and quickly throwing up template websites with little to no value/unique content. These sites will not maintain these PageRank values as soon as Google realizes they have changed ownership and content. There are also many ways Google can identify that these simple sites are selling links.
5. Make sure your links come from US based and targeted websites if your entire target market is in the US.
6. You really don’t want links created by spam software. Most of this software leaves a footprint and it is quite easy for Google to identify.
7. You really don’t want links from junk websites. These are sites like affiliate sites that have little or no unique content.
8. Don’t post links to spam sites relating to the three P’s (porn, pills and poker). Period.
9. Avoid the temptation of posting links to a forum that the owner no longer effectively manages and that is full of spam.
10. Avoid the temptation to add and publish your URL in link memes. A meme is a growing list of links that passes from one site to another for the sole purpose of sharing links and pagerank across the list of participants.

These are just ten of the types of links you don’t want, but this list is not all inclusive.