from 1.7.x to 1.8.x: client/server encryption protocol has been enhanced

  -- 1.7.x clients can connect to a 1.8.x server

  -- 1.8.x clients can only connect to a 1.7.x server, if they
     are built with --enable-encrypt=1



from 1.6.x to 1.7.x: things to watch out for

  -- the log server drops root privileges after startup; it needs a logfile
     directory with write access for the unprivileged user now

  -- the PID file does not double as lock for the log file anymore; the
     log file has its own lock now (same path, with .lock appended)

  -- by default, the HTML status page of the server is in the log directory 
     now; this allows to make the data directory read-only for the server