As some have said already, alot of dilla's music is an acquired taste. In the sense that a casual listener may hear one beat from him, and although they may think its hot, they probably wouldnt care to listen again, or explore the other tracks he's made. but for one to see why he's special, they would have to study and internalize his entire body of work.
sonically, he's made so many transitional changes that it would be a disservice scrutinizing just one segment of it. however he can be considered your favorite producer's favorite producer. A majority of the heavyweights (havoc, premier, dre, kanye, just blaze, pharrell, pete rock, q tip) have acknowledged his greatness and have given him the nod. he also, imo, had some of hardest hitting drums. thats basically a result of him eq'ing the hell out of them. they also had a human feel to them because, unlike many other producers, he wouldnt quantize them at all.
yea i guess one can make the argument that he's slightly overrated (although i would disagree with that immensely). but i think a majority of that argument relates to his death. an artist's death can sometimes propel them to a mythical-like status. with every artist's posthumous legacy, overzealous fans can often get carried away with too much approbation. however, that shouldnt negate from the mark he's left and the impact his sound has had on modern producers like flying lotus, robert glasper, black milk. even outside of hip hop, his influence is felt. indie rock/shoegaze bands like the horrors, golden silvers, and the mystery jets have mentioned how much of an effect he's had on them