What about the one I seem to be complaining about all the time!(ha,ha)
Breath response. How quickly a reed sounds after blowing.Perhaps under the same heading the air capacity. some melodicas take a lot of air in general, some not so much. Some play single notes fairly easily but if you try a three or four note chord it robs too much air.
Along those lines for the two-handed players what about balance? Does the lower register drown out the treble or vice versa.
Also what about tone color? Some are very reedy, some honking and shrill some softer almost flute-like.
Just some suggestions. I like this idea of criteria ratings. Perhaps Troy could find a way to work it in to our review section where we have a 0-10 rating for each of the criteria?