Noting a useful resource.
Blog post by Brian Solis on 'The Science of Retweets on Twitter', examining a report by Dan Zarrella.
This is an overall analysis of Twitter, which provides some possible benchmarks for comparison when focusing on a subset of Twitter.
He records the most retweeted words and the least retweeted words - noting that retweets require a slightly higher reading age than Tweets.
Punctuation is more common in Tweets than in retweets - except for semi-colons, which show up as the 'only unretweetable punctuation mark'.
And retweets vary by day of the week and by time of day - with Friday evening being a top time for retweeting.