(1) Colour.
(2) HDR-2000T has 2 USB ports, the HDR-1800T has 1 USB port.
(3) The HDR-2000T standard remote control (RM-I09U) has a dedicated button for switching the remote to TV mode and a dedicated button for switching the remote to PVR mode.
The HDR-1800T standard remote (RM-H06S) does not have a TV mode. This frees a button for 'WIDE' and another button for 'TV/RADIO'.
(Both units can use the other's remote.)
My combined manual for the 1800T/2000T shows the layout of the HDR-2000T remote. For the layout of the HDR-1800T it erroneously repeats the diagram of the HDR-2000T remote's layout but the description appears to be correct.