I went back and forth with Google for a while after they labeled one of my sites a "bridge site." It's an established site with thousands of pages of unique content. It's definitely not a thin affiliate site.
It was a frustrating process because Google's people won't tell you what's wrong. Instead, they'll offer cryptic clues. It's almost like it's some kind of game to them.
Eventually, though, I realized that the problem was a few pages that contained affiliate links and little else. There were added so that I wouldn't have affiliate listings all over the site. The pages accounted for less than a small fraction of a percent of the pages in the site. I deleted those pages (and added affiliate links all over the site) and Google's happy now.