What favorite feature(s) do you miss when working in other languages?

Coroutines, cooperative threading using fibres, type classes, generics, type deduction, easy interface with C++. The functional style, pattern matching. Flow based programming using 'chips and wires' abstraction.

It has many other interesting capabilities, for example, the ability to change its own grammar, that is rather too much, not for a pleb like me. It has unique (linear and affine) types too. It is really quite a handful.

Go did bring coroutines back into limelight but Felix predates Go by a margin.

Skaller, Felix's author, used Felix as a playground for novel language design ideas, so it was always in a state of flux.