The type system. Memory safety is probably what you hear most about rust, but that's (for me at least) mostly a side effect of it's incredible type system. Not having null, rust's powerful enums and traits make life so much easier. If you get data from the function, you never have to worry if it's different to what the function says it might return, it will ALWAYS return data in the correct format and you WILL have code to cover every single case of that data. I also love Rust's incredible range of "level", going from interacting directly with hardware to writing web servers like you'd do in javascript through libraries like actix_web. Now granted, if you're going to the lowest levels of rust by using unsafe, you lose a lot of the guarantees that the type system normally provides, but even unsafe Rust still has some of the strictest memory management and typing rules for low level programming languages. The ecosystem is also pretty nice, with major crates already established for most problems you'd need a library for. I also feel like I actually prototype faster in Rust than in other languages that are supposedly easy to throw a prototype together with, but this may just be a case of my brain being to oxidized to write hacky code.