Google have completely revamped the Google Groups website and given it a more Web 2.0ish look-n-feel. In the process they have also added some new features some of which are even borrowed [inspired is a better word] from Yahoogroups.
Like Yahoo discussion groups, Google group members can now upload files for sharing with other members of the group with the total storage limit set at 100 MB.
The new UI has received a major facelift and GMail users would love it since it's more or less a clone of the GMail message view interface. Infact, there's a high probability that Google may be migrating all their apps to a standard UI as they have also done that for Google reader recently.
Another useful feature in Googlegroups is the ability to create webpages associated with a particular discussion group. The Googlepages creator like WYSIWYG interface will help you publish the webpages and members will be able to leave comments on these pages. So you can post an interactive classified ad of your house or old TV within the group members easily.
Googlegroups now color has themes to customize the look-n-feel of your groups. Plus the member profile section accepts picture thumbnails, short bio and even a personal quote.
Yahoo! may have to deliver something new very soon.
New Google Groups | Old Google Groups | Official Announcement