1. Readable and debuggable macros. Blockchain devs use them heavily, and it's virtually impossible to understand what's going on for a person, who wasn't involved in the development of that code with macros.
2. Floating point operations reproducibility across environments. When it comes to finding and hashing the consensus among many machines about floating point calculations, Rust gets ugly: in fact, as a developer, you'll have to use either integers or a made up fixed point numbers instead of floats to ensure that calculations are identical on different machines. You need that because hash function will be completely different for two numbers, which are even slightly different.
3. Zero-exception safety mode checking at compile-time. Blockchain code should run forever and never halt. Because if it does, the whole blockchain is at risk of stopping, and some chains may never recover from that. So, just as good as Rust prevents poor memory management, there should be a special optional compile-time check, that prevents runtime exceptions and halts, and forces the developer to wrap all possible points of exceptions in nice workarounds.
Blockchain devs are the best Rust adopters out there, we're excited to see how the language is becoming defacto a standard for web3 development.