All of my mails are newsletters containing 10-30 links, and more than once I've found the mere inclusion of a single link to a certain domain can get something into spam versus a version without that link, often with no clear reason why (domains that are particularly new are one marker, though). Or.. how about using a Unicode 'tick' symbol in a mail? That can get a reputable sender into Spam versus a version without the same single character (all double tested against a clean, new Gmail account) :-) Or how about if you have a link title that includes both ALL CAPS words and ! anywhere? Your risk goes up a good bit, but just go with one of them, you're fine..
I now have a playbook based around numerous findings like this, some based on gut feelings looking at the results and some truly proven, and even with my solid reputation as a sender, I'm having to negotiate a lot content-wise each week. But do I like it? Yeah, in a way, because it's also what stops everyone else being a success at it.. Gmail sets the bar high! :-)
(Oh, a bonus one.. include a graphic over a certain size? Your chance of ending up in the Promotions folder just leapt up. Remove it, you're good. It doesn't seem to be swayed much by actual content. So I've stopped using images where at all possible now and open rates stay up because of it.)