From what I am seeing, Google is going all out to stop link spam.
Panda deals with spammy link sites, thin content, buying links, and similar.
This has caused a severe drop in traffic, across the board.
I have a graphic software sales site that is 99.99% organic. (I may post it's link sometimes but the vast majority of links come from visitors giving tutorials. There are also a lot of hacker and freeby download sites that link to my downloads.
Panda cost me a lot of (spammy) links and a large drop in traffic.
Penguin, on the other hand wants to eliminate non-relevant linking.
Penguin was a recalculation of the effects of links on SERPs with non-relevant links removed.
(Using the new, relevance based PageRank algos)
The links on non-relevant pages were also checked for anchor text, and this was used as a basis for penalties.
Google's Matt Cutts tells us that things are going to get tougher.
I think he means that less and less leeway is going to be afforded off topic linking.
How is this going to affect us? How can we continue to build links?
We must be more careful.
Drive by link spam is a major consideration.
Google has the capability of judging the content of a "Thank you for a wonderful post" type of spam and hammering the poster's link.
Another consideration is the use of signatures with links.
If your signature link is off topic then it would be better to write it in a plain text only format.
@ domain (dot) com
One of my clients was a forum poster in topics that did not relate to their website.
When posting she always made certain that her site was linked.
The majority of her links were built this way.
Panda hit hard and even the domain name's keyword phrase is not showing in SERPs.