Here’s how it used to look.
And here’s how it looks now.
The main thing I like is that you can collapse sidebars you don’t use. As with most Google updates, it’s pretty non-offensive, and welcome.
Official Google Reader Blog: Square is the new round.:
Google is all about speed, both under the hood as well as in the user experience. So, in order to make Reader act and feel more speedy and responsive, we’ve removed some visual clutter, simplified some features and given everything a bit more breathing room. Out with the old rounded corners, drop shadows and heavily saturated colors — in with a softer palette, faster components and a fresh new look.