At this year's Google I/O, Google introduced a unified chat platform under the Hangouts branding. For most, this is a great thing, however, for some, it's terrible. As discovered by danib62 on Twitter, switching to the new Hangouts removes the ability to use Google Voice from Gmail.
If you'll recall from last summer, I wrote about traveling with technology. While abroad, I made extensive use of this Gmail Voice calling feature to make free calls home. Removing it seems like either a gross oversight by Google or a directed attempt to shy away from the Voice product.
Luckily for those who use Google Voice in Gmail, Google is allowing users to revert to the old Google Talk. This doesn't mean that Google won't eventually force all users over to the new Hangouts, but it does help those who accidentally upgraded without realizing it takes away the Google Voice ability.