Two of the big things are:
it's wacky cockpit layout. The engineers desinged a lot of three position switches to reduce the amount of switches in the cockpit. It can be tough at times to tell what position the switch is in. It would be disastorous is some of those switches are put in the wrong position is certain situations. Half of the circuit breakers are located on the captains side and the other half on the FO side. The metro has a pretty big cockpit compared to other aircraft. This makes searching out ciruit breakers to push in or pop out on the other side of the cockpit difficult to find and reach when flying single pilot. It's even worse at night.
it's slow control response. The metro rolls really slowly due to the tip of the ailerons being located 3 feet inbord of the wing tip. It's not so bad at cruise, but it takes a lot of input to make it do what you want it to do at slow airspeeds. Its not uncommon to have to go nearly full aileron deflection (with both hands on the yoke because the controls are heavy) either way while doing approaches in gusty conditions. I've heard pilots compare the metro control inputs at cruise similar to a 727's control inputs configured to land! It also has a small rudder for the size of the aircraft. You have to do everything right to have an successful outcome after losing an engine of takeoff. One mistake and you're likely not to make it.
It also has a skinny wing and cruises at decent airspeeds, but could probably use a few more horses.