I say this as a Rust enjoyer, but I think what most people mean when they say this is that there's a lot going on, especially in the function definition syntax. When you start adding in lifetimes and generics with bounds, async, &muts, where clauses... it really does become unreadable. I don't really see a way to fix this without making the syntax even more verbose, or aggressively simplifying the type system to the point that function argument inference becomes viable, but then you might end up in a situation like Swift's where type checking a simple expression takes an excessively long time.

That and the colon-colons (::), probably. Those can add a lot of noise.