MIDI has permanently changed how we all write music ITB (In The Box). Apart from maybe the guitars, which still have no convincing sample libraries (my opinion). We can comfortably say that we can tackle almost all musical instruments imaginable with close to real-world articulation using our good old friend Midi.
There is definitely a lot one can do with Midi while controlling and playing back sample libraries. But the most full proof and the ones I use the most are Velocity which controls how hard a particular instrument is hit/played and Expression/Volume, which controls the loudness dynamics of an instrument.
Just by properly using these two controls, you can start to get very convincing music parts, and for the majority of times can trick the untrained ear to believe it’s been played by a real person.
Ofcourse there are more Midi controllers available to the tweak-heads out there, but by just mastering these two, your compositions will sound 10x better then what they were.