Size and service.

On name brand carriers, that still offer business and international first, the seat - or suite - is bigger, check in is better, food, etc. is better and prices much higher (and generally not worth it to the mostly frugal FT folks).

In general, international Business class is much better than domestic F. There's also Premium Economy on say British Air, which is a different class of service than Economy. TK's Comfort Class has roughly 2X the volume of space as compared to their (or any other) economy class.

The seat size or various carreirs can be found on Seat Guru. The service is best inferred from reading various threads.