i have greatly enjoyed diablo 1, played some diablo 2 untill the patch changed kille dmy chars repeatedly (no skill resets). and i played diablo 3 with my wizard up to lvl 58 (60 is max) in hell difficulty.
standart difficulty is just for fun... kill them all.
nightmare is getting intresting and will teach you to avoid dmg. but still easy going.
hell is where it starts to get difficult and you switch from dps to max 2 dmg skills and the rest is just for survivability.
inferno.... well havent been there but way more harder.
general i dont like the few skills i have avalible. often i run into a situation where i just would have needed another skill but you cant have them all that the same time.
well it is diablo and i already feel the same things that have drawn me out of wow. the endless grid and no end in sight. always only looking for the next little better items. yes perp has a lot grind too but there you want to build something and not just throw away the old stuff.
you realy see that the makers of diablo (blizzard north) are no longer there.
personaly i dont know if i will stay till 60. maybe cause a few friends are poking me to get there.
what i miss are the masses of monsters to kill like in diablo 1+2. here you mostly fight smaller groups that are harder. so a bit of the diablo feeling is lost.
as a casual game ill say nice, its ok, makes fun. but as a dedicated game it will get boring very soon.