I'd say the core is that anyone could link up and start talking with other parts of the network; the arp/ip technology was the stepping stone of course, but a key differentiator from previous machine networks[1] is that you can just hook up to the internet, it was truly decentralized from its ideation.

[1] ie. stock tickers, terminals of time share mainframes, or see plan 55-A for a wider interconnected switched network this russian idea