One nice thing the borrow checker enables, besides GC stuff: You can guarantee that you never keep a reference to the thing a Mutex is protecting, after you unlock the Mutex.
Replying to my own comment, and very much related to that, another is thread safety. In most languages, if I have a type with some method that e.g. increments a member variable, that method is non-thread-safe by default. (Modifying internal data structures can be even worse of course, depending on the language and the data structure.) In Rust, such a type is trivially thread safe by default, because the languages just won't let you modify it from multiple threads without a Mutex or similar. When you want types that can be shared and modified without locks, the author of the type has to take steps to implement this (using atomics, lock-free data structures, etc). That means that there's generally no need to trawl through docs or source code to find out whether a method is thread-safe or not. The type itself knows.
