ALT SEL seems to catch quite a few people out. It's not a terrible system, just appears very different from the Boeings and Airbus systems. Young pilots with it as their first type seem to have fewer problems with it. The fact you can sometimes knock the ALT SEL out by moving the pitch wheel as it begins capturing an altitude is the biggest thing, but most operators SOPs are reducing the impact of this.
As for the common type rating, the Q400 should have been a fantastic aircraft, instead it has controls very similar to the previous models with wildly different systems behind them. The standard thing heard from 300 pilots converting onto the Q400 is "....but on the 300 it did this"
I understand it is related to beans and counting somehow, but a Dash 9 with autothrottles (it's a FADEC tubine so it's just a case of making the power levers move if you want to be fancy), 3 axis autopilot, heated leading edges instead of boots, a toilet at the back of the aircraft instead of the front (ok so on the plus side the F/O knows when the toilet needs dropping, but it still stinks), the ability to have red and white strobe lights on and perhaps a repeat of the engine torques on the Primary flying displays