It's easy for random people on HN and elsewhere to armchair quarterback, and act as if they know for sure what makes Twitter tick. But they don't have access to the data, and they have no experience building or running a massive social network. (The same goes for me. Take all I say with a grain of salt.)

My guess is there is no single answer to what makes Twitter great. It has so many niches and sub-niches that it's completely different for most people. It's a bit like Reddit, but without formal or visible boundaries between communities.

And the network effects are so strong at this point that it's hard to unravel. Musk can make a million mistakes, and Zuck can launch a million alternatives but they aren't going to succeed in convincing the most powerful Twitter users in each community from abandoning the audiences they've cultivated. And as long as they're there, everyone else will be, too.

Twitter got great by serving their users. They observed behaviors and canonized them. Using @mentions, the word “tweet”, hashtags, re-tweets, quote-tweets were all emergent user behavior that twitter codified and amplified. All without getting in the way of the interactions.