Needing to use two separate third-party crates (lazy_static and maplit) to have a global constant-initialized hash table is the one that surprised me recently.
Sure, maplit's hashmap! is just a convenience macro, but then so is vec!. Hashtables are a pretty fundamental data type and they deserve to be easy to write constant initializers for, just like vectors.
'const fn' seems like a red herring -- I don't want to write const functions, because I don't want to write code to initialize data structures at all. I want to write representations of data structures...
Sure, I think it's a useful crate. But it's certainly not "Need[ed] ... to have a global constant-initialized hash table" that's all.
const fn isn't a red herring; you won't be writing const fns, you'll be using them to do the actual initialization. If maplit would use them internally, for example.
Incidentally you might like the phf crate, depending on your needs.