Nix being an expression based functional language, it doesn't really make sense to have something like let x=y; since this looks to most people like a statement (i.e. a line of code that gets executed as part of a sequence). This doesn't exist in nix—instead you have expressions that get lazily evaluated, possibly out of order compared to what you'd expect. let x=y in makes it more clear that the variable binding you're doing is only in scope for the current expression, which reads something like "let x refer to y in x + 3"

The function definition syntax is unusual but definitely not unintuitive imo. It captures the simplicity of the function semantics of nix—a function is just a mapping/transformation from one value (or set of values) to another. I don't think it's too much overhead to learn that they use : to mean this instead of =>

In terms of why they picked this syntax, it follows the traditions of other functional languages such as the ML family, Haskell etc.