The combat was fun and varied, D3 constantly gives you skills and modifiers for these skills to keep it fresh. But it comes at the cost of being quite overpowered throughout the game. The skills just look and feel better in D2, like compare the mage/sorceress class for example.

The aesthetic is way off in D3 it doesn't look as dark and atmospheric like the first two games.

I don't think the world is fun to explore, not enough things are procedurally generated in D3 until you get to rifts, but rifts are not as fun as "playing through the game" like you get to do in D2.

There are far too many difficulty modes, it's like they just gave up trying to balance anything and said let the player try to balance it.

Everything is simplified, you don't need things like keys, scrolls or potions. The potion is just another skill in D3. You don't really lose enough by dying to care about dying.

I already mentioned aesthetics but graphics is a slightly different issue, D3 doesn't look very good, D2R looks far better.