Diablo had a pretty straightforward gameplay and storyline with no real 'quests' except for stuff like Leoric, Sir Gorrash etc. Diablo had a very simple item & inventory system. Had 17 levels (21 including Hellfire) that are connected, and there is no location system
Diablo 2 had a variety of quests, locations, item types, twice as much classes, a skill system, somewhat advanced graphics, advanced mechanics and such.
Diablo beats the shit out of Diablo 2 easily. The charisma, the straightforwardness, simplicity and the artwork is beatufiul. The music is beautiful too. Diablo 2 seems too heavy, makes you take a break after a while of playing. Whereas, Diablo will suck you in. My record of playing Diablo II was about 4 hours, with the first game it was 13.
That's my opinion.