I'm about to write some top 10 SEO mistakes but after searching in the SEs, there are already some materials like the ones here and here
but the latest one was made by Peter Da Vanzo
.

Let me add some more of the latest SEO mistakes that has a major impact recently.

1. Excessive subdomain use.

Some people have discovered that by using keyword.domain.com, they can be indexed faster. Well, this worked until google's BigDaddy came along.

2. Linking to a totally unrelated site

In my test bed, I did some interlinking with some of my test domains from totally unrelated topics. Just recently, it got penalized. Now these domains has a PR3 and below and not much backlinks and thus it lost a lot of its indexed pages.

However, if the sites interlinked are big networks of domains with tons of backlinks such as Creative-Weblogging network, the effect is not that great. In fact, I don't feel any effect at all. If you check out the right side of this blog, you'll see links to other blogs which are totally unrelated to SEO but still it doesn't seem to affect each blogs.

That's the good thing about google's algorithm. If its an established domain and is seen as an authority in its industry, and has great content, then you can be sure that despite google algo changes, it is still there.