A big one that I feel is under appreciated is how Swift has rooted out nearly all passing around of untyped data, untyped dictionaries, casting without checking, etc in Apple platform projects.

I don’t mind Objective-C when I’m the one writing it and can ensure that the code is responsibly written, but it wasn’t unusual to have to work on existing Obj-C codebases littered with untyped data, casts, etc some amount of which was inevitably erroneous and only seemed to work correctly. Chasing down the origin points of data, fixing a laundry list of method selectors, and adding in checks always sucked with the compiler doing little to give you a hand. With Swift, even in codebases with poor hygiene fixing things is easier since the compiler will yell when something’s wrong.