Count-ins - by which I mean saying, e,g, 1-2-3-4 1-2-3 before starting a tune: some folk just can't do it even though they might be able to play the tune once someone else has started it: because they can't feel the rhythm in their heads.
Oh, and don't get me started on stamping - I absolutely hate it. Again - feel the rhythm in your head, don't inflict it on the floor - and other people's ears. (I have seen some musicians stamping along while they are playing, totally out of time with what they are doing!)