Dude, you must be new to it.
In D2 we killed monsters to get runes and make wacky new builds such as bearzons and whirlwind assassin and ghosts and... I remembered I had a archive in my PC with a list of 230 character builds for D2, from poison fairies to singer barbs. They were not always optimal but strangely enough (given immunities and other monster properties) some wacky builds were better at farming certain items then mainstream builds. And that actually mattered because some lower level items were also useful.
Also in D2 had a non-linear progression. Learning to beat nightmare was not the same as learning to beat hell. You had to build around immunities, some skills didn't progressed at all and you needed new ways to make then worth it (such as necro needing specific call to arms item to make skeletons good), not all character approached each item equally (so you needed to learn many new things when changing classes, like break points for zon IAS jab skill and how to make a damaging auradin, while in D3 it comes down to solving the same mathematical equation max DPS restricted to certain EHP level).
I still play D2 and every time I play it I learn new things. Getting items in D3 to jump from mp5 to mp6 and from mp6 to mp7 feels like the very same experience to me... I'm a guy who like new experiences =p (in 4 years i've never had the same meal twice).
I'm not saying D3 is a bad game or you're stupid from liking it. I just don't like how things turned out in the end and that's my preference. I just read those board once in a life time to see if the expansion will fix the game for me =(