Those who are currently using a 1.x version of Thesis are advised not to update to 2.1 unless they are ready to deploy a completely new design (or stick with the built-in Classic Responsive Skin), as Thesis 2.x does not inherit any of the design settings or customizations from 1.x.
(SEO settings and other post/page data are retained, though—we’re just talking about the design here.)
Why? Because Thesis 2.x was literally re-written from the ground up—it’s an entirely different platform, with an exponentially improved core that offers far more power and flexibility than its predecessor.1
So, if you have a highly customized design built in Thesis 1.x to which you’re fully committed, and you don’t want to expend any effort to recreate it in 2.x, then you may not be ready to take the leap yet.
But when you are ready, Thesis 2.1 will be ready for you!
1 One notable advantage of this fundamental difference between the new versions is that you can easily switch back and forth between them, allowing you to “test drive” 2.x without losing your 1.x settings.