My favourite thing about swift is actually the core team. Not only are they excellent at jumping in to support users, but they are also very good at looking past community bias' through feature evolution and taking a very pragmatic approach to how swift should evolve over time. Sometimes there are issues with transparency but I believe that is due to being hamstrung by Apple policies rather than them not wanting to be open about certain things (it must drive a few of them crazy).
If we are talking about the language itself, my vote goes (like some others on this thread) to enum, I really miss its flexibility when moving to other languages.
There are a few, but the one I find myself missing most often in other languages is the extension system. Of the languages I work in, Swift is the only one that allows you to define static extensions on anything (or at all for that matter) and even define extension initializers. I also find the extension block syntax the most intuitive.
the flip side of this is it makes it immensely difficult to build source/documentation tooling for swift (compare with languages like java where the tooling is far ahead of ours). we also have a problem with so-called “dark API” that’s completely un-documentable and un-indexable (see explanation here), and i’m not sure if there exists any parallel to that problem in any other language.
it’s possible that this is one of the factors that has slowed uptake of the swift language among large organizations.