I literally downed diablo in less than 5 seconds on sorc, granted it is quite harder on a melee class especially at this stage without gear which they rely on more than anyone else, but if you think he is hard you will not have a good time with the rest of the game.
he is still very vanilla and not hard at all, wait until you get to Baal Hell, i believe that is the only vanilla act boss that has its own mechanics, the rest you faceroll.
Vanilla acts are easy, even the new mini bosses Median devs put into vanilla is done without any issues.
Once you get to Baal hell let me know what you think of normal diablo then. And remember Hell Baal is the easiest major endgame boss in the game.