D1: Limited gear choices, no running, mana shield + spam pots = god mode, every pub game swarmed by spammers/townkillers, corpse popping, no variety between different players of the same class.
D2: meaningless resource system, badly designed synergy system, hopeless search for runes you'll never get to make that stupidly OP runeword that everyone duped, thousands of baal runs led by a teleporting pally bot, bugged gloams, utterly pointless PVP/griefing, spammers. I could go on for a long, long time.
The first two games weren't perfect, trying to pretend they are will just make you more disappointed when D3 isn't the game you remember. The first two aren't the games you remember either, for that matter.