everything above is true... but you also have to follow the mood of the times, different era's different sounds....In 80's Hip Hop, B-boy sound, 90's Gangsta,hardcore,mobb,pimpin, late 90's Bling Bling rap, and ect...ect...
I just had say that cuzz I met this old school producer that was trying to sell me some beats that sounded like it was from the mid 90's....LOL