ATR is Airbus, hence why there is a much closer commonality with avionics/FMS etc, especially the 600 series. Despite they are both "jets / turbines", it`s the engine management of heavy turboprops which creates the largest learning curve, the propeller creates a much longer list of potential problems, just take one look at the engine section of the QRH for an ATR compared to say a 737. Engine flameout scenarios can be complicated, handling & procedures during EFATO is a very different story to an Airbus or 737. Icing procedures can also be very complicated on the ATR.
Basic hand flying and normal procedures are pretty much the same, not too much difference in approach speeds actually. However on the other side of the coin, energy management during descent on the medium/large sized turbofans is a little more difficult to get used to initially.
They`re just slightly different machines, like Geotracker said, it`s just a case of getting used to it. Some pilots just adapt more easily than others. As we have seen, it`s not so much the difficulty but the perceived training risk from recruiters which often restricts movement between these two categories of aircraft. However, I`m sure more opportunities for turbofan pilots to progress their career on to similar sized turboprops will appear in the near future.