Is it possible for some links to bring your site down in ranking? For instance if you were linked on a site that was known to be spammy or to have low quality content.
no it will not hurt your rankings....but yes, if you link back to these sites, your rank and PR will definitely go down. moreover, if spammy sites are linking to you, you will marginally gain from these links
External links withlow page rank would definitely harm yours page rank .But I have a formula that can help you to get on first page of google without any tension.The simplest formula that I have followed and designed and got #2 in 15 days. The formula called Google Ranker. Go for it! Ranvijay
Sorry, but that's totally wrong. External links pointing to your website certainly won't hurt any rankings because they have low pagerank. If you are spamming hundreds of social bookmarking sites and gain masses of low quality links, without having quality ones as well (pagerank doesn't mean quality) you may run into problems, but a few low quality links are no problem at all.
The only inbound links that might hurt your rankings are paid links and ONLY if Google can prove you (and not your competitor) paid for them. This means about the only way it will hurt you is if you were stupid enough to be logged into Google Webmaster Tools and bragging about paying for links on Googles help forums. Outbound links, however, are different. Linking out to bad neighborhood sites (sites known to violate Google's Webmaster Guidelines) CAN get your site penalized... So always include a rel="nofollow" on any outbound link that you are not willing to vouch for.
Inbound links can hurt. Google has never stated that competitors can not hurt you (read their guidelines carefully). That said, there is little that can be done for it if someone is gunning for you. Worry about what is under your control.
Canonical, sorry you are wrong, the nofollow tag is no longer recognized and does not block link authority from passing anymore. I have seen sites get penalized for a sitewide link buy on a massive site. I have seen a fairly new domain get penalized for a sitewide link on a VERY authoritative and relevant site (few hundred pages) and a very aged domain with a large link profile get penalized for a sitewide on a site with 850,000+ pages. So it can vary by domain but I am willing to bet that if you wanted to "bowl" over one of your competitors with a sitewide on a HUGE site using their main keyword as the anchor text. Other than that it is hard to get penalized for linking. You just need to stay away from sitewide links.
As long as you have some good sites linking to you too, I don't think bad sites linking to you can hurt. If that was the case, then people could get a bunch of bad backlinks to top ranking sites and get their rankings lowered.
This is true. Inbound links from spammer site never hurt your site ranking but it should not be 2 way link. Bcos 2 way link (Link Exchange) becomes with your control. But 1way external link can be added by any site Owner who like or unlike (competitor) your site. this things not under your control. So ne need to worry about the inbound links which are linking you one way.
Not really a negative effect but it somehow relates to it, post links on relevant sites will help you but when you post on non-relevant site, I guess Google would look at it as more irrelevant link...