This is a biggy for drummers and most modern music. The existing input system is very cumbersome and very non-standard [or at least unlike any standard I have ever come across].
MIDI mapping
It should use the same conventions as virtually all the other scoring programmes, DAWs, drum charts, magazines, tutorials etc and use GM drum mapping. At the moment it is a frustrating game of hunt and pack to find out which MIDI notes are mapped to which drum.
This is particularly crucial when inputting drum parts from a MIDI keyboard or electronic drumkit.
Drum notation / scorewriting
Musescore uses a default drum map that you can customise / save / load. Each drumkit MIDI note is mapped (or unmapped) to a specific position on the staff, a voice (which also controls the stem direction) and a note head.
When writing a drum part simply place a note on the staff, there is no need to select voices or note heads!
Being able to save your own custom drum maps would be a bonus.