I'm the rookie out of the guys I have met on my home water, and they are all accomplished casters. I am truly too humbled to be critiqueing they're casting. So I'll give an account of what MY biggest errors are.
#1) I try to overpower the rod on the forward stroke, not letting the rod do what it was made to do. I instinctively try to "Power throw" the forward stroke and it is HELL trying to force myself to wait for the D loop to form, not creep forward (which shortens my forward stroke, I know, Iknow.....) and slowly but increasingly come forward and slightly down with the rod to a nice underhanded "Snap and Stop", ending high at 11 o'clock. (Thats more than 1 thing, I know, but they're all part of the forward stroke so I'm calling it 1 thing)
Other than that I'm doing OK, and when I shorten up, slow down and tie some yarn on (or when no one is looking and it doesn't matter) I do fine, so there's hope for this old dog yet!