Comments on Factors Negatively Affecting the Value of an External Link:

• Adam Audette
  – All killers. The last one is a grey area...but a major factor. If a link is determined to be paid, it will normally be filtered out from the site's link graph. But there are occasions when a serious penalty will occur from too many paid links.
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  Chris Bennet
   – I don’t know what measures Google has taken to algorithmically spot low quality paid/rented links but it would be very easy to build a tool that could spot 80-90% of the crap without breaking a sweat.
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  Hamlet Batista
   – Links from banned sites are pretty much worthless.
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  Todd Malicoat
  – Most links won’t hurt you, but if you put significant effort into obtaining a link that won’t help you, you’ve negatively impacted your bottom line. Make sure you are hunting for links that matter.
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  Ian McAnerin
   – Links are not a rankings factor – trust and topic are. Links just represent this. If you can show that the link has little/no trust or is unfocused, then it will not be worth much. If you can show it has neither trust nor accurately indicates the topic, then there is no reason to count it.