My use of google reader has slowly declined since I have finished the uni course, but I still see the need to follow smart people, and google reader still gives me that instant hit of something to think about.
About a year ago Google Reader updated their social features to first let you star an article, then star or share, and finally star, like or share. It seemed a bit confusing when you would want to do each, so I generally found myself either checking all of them, or none at all. I finally figured out it's all about context:
Star - It's of personal interest: You might star something as a reminder to get back to it, because it's important to you personally, etc. I'll generally star things I've responded to so that they are easier to check followup comments, or make the whole conversation easier to find when I'm searching for a previous article.
Like - Attribution back to the author and other readers of that feed: This is an easy way to communicate simply with other readers of the same feed. Liking every single article on that feed doesn't really help you or the other readers, it's best used as a tool of discernment. I'm also far more interested in seeing who else liked the article if there are fewer names, and repeat names are worth investigating. Likes are one step above a comment as a way to easily engage with the audience of the article.
Share - Distribute the article to your friends that may not know about it: Share the link with others. Need to check the publicity settings before mashing this button to see where it'll go. I also send my shared feed out to a widget on this blog as a semi-kudos instead of writing up something specific about it.