Rust offers players the tension of DayZ combined with a unique charm to it, along with the best building system out of any survival game in the market. Seriously, every building system should just copy Rust, please.
Player interaction is what makes and breaks this game.
Rust offers a massive playerbase, so you'll come across all manner of friendly encounters, roleplayers, screeching children and memesters, but for every group like that, there's about 5 more that will cave your skull in with a rock or put a bullet between your eyes for legitimately no reason. Except maybe cannibalism. But hey, that just makes it extremely entertaining to watch on YouTube.
PvP is exceptionally intense as well, with raiding and skirmishes being extremely prominent given how hostile most players are.
More than anything though, Rust's Achille's heel is time itself. With most servers operating on weekly wipes, Rust encourages players to log in en masse on Wipe Day so they can quickly acquire the boots needed to stomp on any players unfortunate enough to attempt joining in after them.