Network transparency kicks ass. It's certainly a killer feature for X.
but there are things that could be better.... Like session management. With Windows and RDP/etc or VNC I can disconnect and reconnect again later without interruption. That's something you cannot do with X (at least the way it is now).
SPICE kicks ass. As far as I can tell it's much faster then even ICA, which blows X and VNC out of the water. I wonder if there was someway to integrate Spice into Wayland.
Like maybe have a Gallium driver that outputs to Spice instead of to VGA out or something crazy like that.