D2 allows for a myriad different builds, many of which are quite creative. It could be that I prefer playing those "half-assed builds" because they are more interesting than the cookie cutter builds (and yes, those "half-assed builds" can indeed complete Hell difficulty if you know what you're doing). That diversification is not available in D3 due to over-simplification. So to me, D2 will always be superior to D3 for that reason alone. For me, one of the most important aspects of an ARPG is build variety. D3 completely and utterly fails in that regard.
So for me, I'd love to see a solid, stable version of D1 and D2 running on modern systems.