In the past I play only with my friend in D2 who always wait me and have pretty similar skill. And it was fun.
But playing with random people is not fun at all.
I tried play Nigthmare difficulty with my Assasin 37 lvl after finishing main campaign and in the very first session one single Amazon can kill solo any group of monsters in blink of eye with her ability Javelin. There was also other 4 people in this game, so mobs should be strong enough. Honesly it felt like I play with some cheater.
Without proper balancing between classes this gameplay makes no point. When I play coop I want to be resourful, not somebody who just running aroung and gathering loot.
So I see clear issue between melee and ranged classes\abilities.