I've taught many guys coming off other types (both jets & turboprops) and they've all said the ATR is the most difficult aircraft type rating they have done.
Just to name a few of the main issues I've observed:
Operating speeds - Even very experienced ATR drivers will struggle to explain the various speeds on this aircraft, in particular when operating in icing conditions.
Icing Conditions - The ATR has a long list of procedures (both normal & emergency) for entering / leaving atmospheric icing conditions, ice accretion, ground icing, de-icing, you name it.
EFATO / FIRE etc - The single engine profile and/or engine fire at takeoff is relatively complicated. In addition, it's actually the lack of performance which is the real problem. If you're heavy and in icing conditions your speed control / handling during the EFATO has to be spot on. Throw in an autofeather failure and you're really in for a treat.
Powerplant - This is not really specific to the ATR as such as I'm sure the Dash 8 will be similar in this respect but the powerplant abnormal / failure section in the QRH is particularly long. There's simply a lot more that can go wrong when you add in those huge variable pitch fans.
Interestingly, the latest OSD (OEB) now actually requires a minimum experience level (500 hours from memory) before even starting the type rating, otherwise the type rating course is effectively twice as long. This has actually come from ATR as they were struggling to get guys through the type rating.