Besides the commenting thing, something I haven't seen covered elsewhere is that I won't have any idea WHO retweeted the thing that shows up in my timeline. The authority (and, indeed, just the personality) of the person recommending the tweet has a lot to do with whether or not I care, or what my expectations for a link in that retweet might be.

EDIT: looks like the screenshot in ev's post was misleading-- it does show who retweeted:

http://www.techcrunch.com/2009/11/10/hate-it-or-love-it-twit...