The materials used, the ease of flying, newer programming language, newer silicon architecture, ease of maintenance, self-recoverable under pilot unavailability, highly efficient high life engine, ease of multitasking for the pilot and so on. I'm not necessarily making comparisons with all the above points, so don't get into an argument. Just describing "modern".
Tejas Mk1 is more modern than Su30 MKI, but not close in capability. As a team, they're remarkable.
Block3's claim to modernity over Mk1A will be fly-by-optics and DSI. I can't think of anything else. Tejas intakes are hidden to an extent under the wings, so again, the advantage of modernity becomes doubtful. Mk1A is more modern in most of the things I earlier listed.