Twitter itself has surveyed the success factors of retweets, tweets recommended and shared by others. To get reliable data the tweets of verified users have been examined by watching the different sharing behavior in the case of varying means included within a tweet. These means are photos and videos, hashtags, links and the mention of scores.
The results are a bit astonishing as hashtags, a basic feature to find news about particular topics on Twitter, are the least important mean to foster sharing. On top are photos followed by videos, quotes and numbers. But the overall picture is more complicated as combinations of e.g. media and hastags may lead to more optimized results, which again differs by topics. To get more detailled numbers read the following summary and the official Twitter blog post.