The new trainsets ("Eurostar e320") are built by Siemens as the 'Velaro e320'. They are factory-equipped with ERTMS equipment (in-cab signalling, sensors, etc), and unlike the Alstom-built predecessors, support both the Germanic 15 kV 16.7 Hz AC electrification, and 1500 V DC used in the Netherlands.

The largest single difference is that the newer trainsets can take different voltage/current combinations from overhead wires. This is important because different regions in Europe use different electrification systems, regions that can now be reached because of new trainsets.

As a passenger, they're taller (the original trainsets were built before HS1 so had to fit the traditional British loading gauge) and more spacious inside (partly due to slim seats), and I think a little sleeker on the outside. But the relevant part is that they have the right signalling and electrical equipment to run in the Netherlands.