I have two sites, one was hit by the first launch of Penguin, the other was unaffected until the refresh. Now it has been blasted. What I did was to try an experiment, and although it was only one site and therefore can't be real evidence, it may be something to think about. I am active on a number of SEO forums, and before the last Penguin refresh I changed my signature link to point to my second site. I didn't do anything else to the site, I didn't get any other links and I didn't add anything or take anything away. But from having 30 keywords that were in the top 50 Google results (many on page 1/2), after the refresh I had nothing at all in the top 50. The ranking of my main site has not changed appreciably, but although Google has picked up the new links to the second site, it is still showing most of them as still being active for the main site. The anchor text was varied, and the links were not all to the home page. Although as I said at the start this is just one site, it does lead me to believe that irrelevant signature links can actually be harmful.
Most of the forum links are nofollow these days and those links that are do follow are more or less spam and so according to me it is more to do with the quality of outbound links from those sites as well.
I am not so sure about that. I use webproworld and V7n and the signature links there are not nofollow, and I would say that they are both well moderated and respectable sites. I have hundreds of links from both, and from others as well. Even if they are nofollow, unless (as with this forum) they can only be seen by registered users then Google is still aware of them.
Then possibly too many links with same anchor text because in forum signature, you cannot vary the anchor text. I have seen a sitewide link had issues long time back. See http://imtips.co/site-wide-links-over-optimization.html
Yes that is a problem with signature links, i.e. you cannot vary them. I've now deleted all of mine from forums that are not relevant to my site that Google can see.
I think that people on forums can work out from the anchor text (and in many cases there is surrounding text as well) in signatures what the linked to site is about.
I came across something recently in Google's Guidelines that I hadn't seen before, under the heading Here are a few common examples of unnatural links that violate our guidelines: one of the examples is: Forum comments with optimized links in the post or signature, for example: Thanks, that’s great info! - Paul paul’s pizza san diego pizza best pizza san diego That could certainly affect a lot of the signatures I've seen on various forums that Google can see as well.
Glad to see that in Google Guidelines. It would help reduce forum spam provided it is read by those spammers.
Links from forums are anyway devalued in Google and so better not waste time in that arena these days.