Diablo 2 is incredibly generic. It's just a case of clicking the mouse, in order to kill as many enemies as possible, to get as much loot as possible.
It's about as mindless as they come.
Diablo 2 definitely isn't perfect but it successfully taps into something very basic in the human mind: instant gratification. You click on stuff, stuff dies and you get loot. Blizzard successfully balanced this mechanic in such a way that you get rewarded through loot often enough to keep clicking, but not too often to make it boring. It is a genius game in that regard.
Also Diablo 2 on normal difficulty is pretty trivial once you get a hold of the game. Diablo 2's challenge comes in at Hell mode, where party design, loot, and teamwork start to matter. Yes you are still clicking on stuff to kill it to get loot, but you are also progressing through a challenging environment with friends.
That challenge resembles old school video game difficulty, aka "unforgiving". If you design a poorly balanced party, once you get to Hell mode your party stalls and you are doomed. This is where Diablo 2 is not mindless, you have to plan far in advance to create a successful party or else you get no where.
There is a reason that many many people spent an incredible amount of time playing this "generic mindless" game. Many of those people who did are not the ones now currently playing COD 2012. Diablo 2 was definitely not the COD of is day as far as level of generic-ness.