Here's a nice little article, with sequenced audio examples, on how slight differences in drum pitch, velocity and timing affect the perceived groove. The title of the article makes it sound more interesting than it actually is, but I did become inspired to try some of it in my own beats:
 
http://www.attackmagazine...choacoustics-of-drums/

Interesting article. This explains what I've experienced for a long time, but didn't really understand. 
 
That was that drum parts played by a real drummer, even if played on electronic drums, recorded to midi, and quantized to exact timing always sounded better than parts programmed with a step sequencer. 
 
Turns out most of the parts played by real drummers always have the variations in velocity corresponding to the groove of the song. Giving them a better groove feel even though they might be in perfect time. 
 
Great to learn and something to keep an eye on in recordings.

this is the basis of toontracks' midi data, and the velocity controls at the groove level, articulation at the construct level.