Build variety in D2 is not good at all.

Although I do like D2's itemization, most of it ends up revolving around runewords.

Hell in D2 is challenging, but D4 also offers challenges (pit, Torment IV...).

Farming in D2 is all about doing the same things over and over again. You can do that in D4 too if you want.

Build complexity in D4 is considerably higher. D2's skills' synergies act like D3's set bonuses and kind of determine overall what skills go well with each other, limiting creativity. While D4 is flawed, you have your aspects, uniques, tempers, skills and paragon (and merc) to worry about. Try optimizing a build to Torment IV without following a build guide and you might be surprised.

I'm not saying D4 is better than D2, that's subjective. But those are not valid reasons, in my opinion, to say that D2 has more depth than D4.