tl;dr: if you're upgrading larger codebases piece by piece, you can now generate v2 API objects from your v1 state (by handing it your v1 `env` object). Requires installing both versions in the same environment (which has been supported since 2.0; see http://www.fabfile.org/upgrading.html#running-both-fabric-versions-simultaneously).
This addition makes it a lot easier to start using Fabric 2 for new code without being forced to update legacy code immediately 👍