TCO (you need explicitly rec annotations), immutability (ref cells, array types), existence of imperative control structures.

Just because you need a red annotation doesn’t mean they don’t have TCO. Most people consider Scala to have TCO, and you need rec annotations as well. Also I said mutation is discouraged for ML languages, not that it hard. Point taken about the imperative control structures, though.

Right, the generated machine code also matters.

Scala has partial TCO, surely not cross recursive calls like Scheme language standard requires and how it is defined from CS point of view.

Same applies to F#, because neither JVM nor CLR have direct support for TCO.