See URL for the release announcement which also includes a list of changes.
""" There may be some problems in the new interfaces and new features which really need incompatible fixes. It is likely that there will be an update in the form of a 3.1 release in the not too distant future, with small but incompatible changes, and if that happens, bugfix-only releases 3.0.x are unlikely. Users and applications which desire better API and ABI stability are advised to stay with nettle-2.7.x (latest version is now 2.7.1) until the dust settles. """ so we wait.
3.1 is out now.