Interesting. This is what is happening to one of my websites now.. After excessive blog commenting, google indexing is not as good as before.
Google will take into account the overall number of bad links and the ratio of bad to good. Google has to protect webmasters against "smear campaigns" otherwise all you have to do is spend a few grand on comment spam bots and forum spam bots to take down a competitor. Hence ratio is how it is judged for sure. That would just logical.
If you were commenting on lots of other sites and the comments were getting removed as spam, it could put you on a comment spam blacklist. A friend of mine said that being on such comment blacklists will also cause issues with Google as Google looks at who is on these blacklists. I have seen some circumstantial evidence that points to this being true but can't be sure at this point, but it might have something to do with it.
Backlinks can hurt you if you keep participating on link farms and blogs with hundreds of spam comments.
I'm not sure why this is even worth mentioning... if your site is participating in multiple black-hat techniques it should be fairly obvious you are opening yourself up to being penalized or worse. "My site is nothing but a bunch of keyword spam, using link farms and dodgy 3-way linking schemes.. can't figure out why Google black-balled me?!??" Bottom line is, if you run a clean site you have nothing to worry about, especially from shady backlinks.
Pretty much true, but there are perhaps some occasions when clean sites can get hit: Domain has pre-existing penalty, site gets hacked, site has downtime, accidentally has hidden text, some other mistake.
The only time your going to have issues with a large amount of "bad links" is if it's a new domain. If Google sees a new domain pop up, immediately followed by spammy links, then Google will penalize you. However if you use an aged domain, then your website can withstand a larger amount of "bad links" without being penalized. Also, a "bad" link is so situational and there are no absolutes. Some #1 ranking websites have over 1 millions backlinks... don't even try to tell me that even half of those are relevant/quality backlinks.
I think that outgoing links to crappy sites influence your webstie in a bad way. If there was such a thing as bad backlinks, people would build spam backlinks for their competitors in order to outrank them.
Bad backlinks won't hurt you that much in the SERPS. It'll definitely make your website look bad, though! Yugo,
I agree new domains are more susceptible, but I've seen evidence to suggest this having a negative effect on aged domains that lack authority, it could be some other factor though. However, given that matt cutts has stated that penalized sites with bad backlinks should move to a new domain as Google mods won't manually go through and discount those dodgy backlinks, then that suggests that even older sites could run into problems, at least with manual reviewers.
There are blackhat companies that actually do build bad backlinks to competitors. I touched on this earlier in the thread:
Great thread guys... one thing I would like to admit.... non-relevant links are also judged by the Google.... it can be more or less useful to break the pattern of incoming links with the same keywords..
For me if backlinks could hurt a a website then I should build backlinks using my competitors url and submit it on link farms and spam blogs.
One of my competitors has been at number 1 for over a year, (it's an awful site), but has a lot of backlinks to forums, links pages etc. They seem like pretty bad links, but the site is still at number 1. Quite annoying!
You can report sites breaking the rules: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=93713