From a drummer's point of view, they have (the good ones, anyway) round-robin sample playback, so each snare hit is actually a different recording of a stick hitting the batter. This prevents the effect known as machine gunning, of exactly the same sound being spat out each time, rat-a-tat. Beats sound more natural as a result. Given that there can be, oh I don't know, maybe ten different velocity level recordings, and maybe six round-robin samples for each, that's.... er... 60 samples being available and used for one snare drum.
The rest is user interface, ease of use, etc.
I have made those figures up but I don't think I'm very far off.