My favorite is uniform function call syntax. In several languages (Nim, Koka, D, …), you can always write bar.foo(baz) instead of foo(bar, baz) and vice-versa.

Another one from Nim is the implicit result variable. Instead of having to do this:

    func sum(nums: seq[int]): int =
      var result = 0
      for num in nums:
        result += num
      return result

you just do this:

    func sum(nums: seq[int]): int =
      for num in nums:
        result += num

It saves so much time and I'm disappointed that more languages don't have it.