Google Reader just had some new functionalities implemented which also contains a 'like'-button:
Have you ever wanted to tell an author or publisher that you appreciate an article they wrote? Or maybe you want to let your friend know that you enjoyed the blog post he shared with you. With a quick click of the mouse (or a swipe of the "L" key — for the keyboard shortcut pros), you can "like" any item in Reader. All "likes" are public, so anyone reading an item you've "liked" in Reader can see that you're a fan. Checking out shared items for people who have "liked" the same items as you is a great way to discover other people with interests similar to your own.
Well let's first go to the description of Google Reader:
Google Reader constantly checks your favorite news sites and blogs for new content. Whether a site updates daily or monthly, you can be sure that you won't miss a thing.
One more thing: as discussed earlier on other places on the Web: 'Like' is completely the wrong expression of content you recommend. For example: if there is a background article about the Holocaust and 10 people like it, it does not mean they like the Holocaust, they recommend it to be read by their network and they want to let others know that the article is worth the time and attention.
Actually, there are a lot of websites with mixed contents, which is impossible to 'like' feed-wide. It's also impossible to rate a webcomic by it's feed; take Explosm. The ability to like certain items is certainly an addition. And who doesn't want to 'like' the news of a newborn baby giraffe?