diberry - 4:27 pm on Dec 16, 2012 (gmt 0)
All the folks on Google Webmaster Forums have been unanimously saying that those errors don't affect your ranks and SERPs. But the evidence suggests that they are interlinked.
Some of those people are just plain wrong about a lot of stuff. If you think about it, why does WMT give you info on the crawl errors and all that if it doesn't matter to Google? I think it's very logical to assume they at least *can* count against you.
As for removing thin content, which creates 404s unavoidably, I've been wondering about that too. I have under 100 404s because of this on one of my sites, and I don't know if it's a problem or not. But I don't mark them "fixed" because they're not - they're meant to be 404s. I think I just have to wait it out.