Some of The Common mistakes done while getting the back links are 1) Getting links from irrelevant sites. 2) Links from over crowded links placed sites 3)Posting the threads that already outdated. 4) Getting Non followable links by commenting.
The only problem i see in seo is on linking to sites that are not related to yours. If someone links to you naturally from a site that is not related to your site, what is the problem? You didn't asked them to put the link? Why should google penalize you? Yes they can penalize if they see some kind of collaboration between you and them from which can result a link exchange deal. Also from sitewide links. If 10 blogs like my site and they link to my, what is the problem? Another non sense, made by seo experts so people will be affraid to make link exchanges with blogs so they can do and rank better. Build links carefully, don't send mass spam emails with links exchanges with your links. Be smart.
A lot of blogs aren't really 'about' anything, so how could links from them be 'relevant' or 'irrelevant'?
Ya, that's true. But it's hard to really judge whether a site is relevant or not. Even if they are related, it doesn't mean that it is more of a "value". But at least you can attract human visitors.
I think some relavent links from good sites are better then thousand of irrelavent link from non-theme based sites....