The big new features are SSH support, the linuxdisk plugin, writing plugins in Rust, and extents. Extents allows NBD clients to work out which parts of a disk are sparse or zeroes and skip reading them. It was hellishly difficult to write because of the number of obscure corner cases.
Also in this release, are a couple of interesting filters. The rate filter lets you add a bandwidth limit to connections. We will use this in virt-v2v to allow v2v instances to be rate limited (even dynamically). The readahead filter makes sequential copying and scanning of plugins more efficient by prefetching data ahead of time. It is self-configuring and in most cases simply adding the filter into your filter stack is sufficient to get a nice performance boost, assuming your client’s access patterns are mostly sequential.