One of the big features of Samba 4.4 is support for asynchronous flush requests for SMB2/SMB3 clients. Samba 4.4 also has a number of improvements to samba-tool, smbstatus improvements, improvements to KCC but still being disabled by default, DNS server improvements, and various other enhancements. The asynchronous flush requests allow SMB2/SMB3 clients to not block other operations when handling flushes.
An experimental feature of Samba 4.4 is support for SMB3 multi-channel, which is a protocol feature for letting the client bind multiple transport connections into one authenticated SMB session -- thereby yielding better throughput and greater fault tolerance.
More details on Samba 4.4 can be found via today's release notes.