Things 1.0rc has a huge list of changes including new icons, the ability to reorder to-dos in the Today list across project and area boundaries, the ability to edit titles and reorder projects in the sidebar (my biggest pet peeve prior to this), fixes to syncing and compatibility issues with the iPhone version of Things, and numerous big fixes. A complete list can be found here via MacUpdate due to site difficulties with Cultured Code. An official mirror detailing the release notes can also be found here.
Things 1.0rc is available at the mirror of the Cultured Code website, since the main site is either being sluggish or not responding at all due to demand. An alternate download location can be found through MacUpdate, along with the complete list of changes. Full price for a Things license is $49USD, but a coupon for 20% off can be used until January 15 - even if you missed out signing up for the newsletter. That code is THINGSPRESALE20.
Things requires OS X 10.4 or later. It is also recommended for those who own the iPhone version to update to version 1.3 once it becomes available through the App Store.