Diablo 2 has my firmly in its savage clutches once again. I am playing the Eastern Sun mod and having a wonderful time doing so - I highly recommend it over playing the regular game online and indeed over doing nearly anything else. It has given me some very powerful insights into game design as I play through it too - it isn't all about killing things and taking their stuff after all. (Yes it is.) One of the most substantial differences between ES and Classic is that Classic has extremely bumpy gear progression and little flexibility. In Classic nearly every item is utter crap but there are a few unbelievably powerful items that are obviously the best thing you can wear. The trouble with this in a game where you progress by killing random monsters and hoping to get a lucky drop is that you end up with a few incredible items and a few junk items that you don't care about at all. When you can find an item that will give you 50% more damage why bother trying to figure out which of many shoddy items could give you 1% more than the other? ES has a much bigger list of items of nearly every category including runewords, uniques, sets and others and takes much better care to make sure that the quality of these items is not so easily divided into 'crap' and 'awesome'.
This has many consequences. One of the most obvious is the constant upgrading of items. Instead of just putting on anything at all while I wait for the insane drop I regularly look for upgrades and rearrange my gear to make sure everything balances out nicely. More of everything is good but I need to make sure I have enough resists, health, mana, pets, damage, magic find, etc. and no item does everything you want so there is a constant shuffle in and out as I find more things. One of the other great consequences of this design is collecting. Since many, many items are relevant I need to have a monstrous collection of items on hand to make sure every character can get an excellent set of gear. As stamp collectors, rock collectors and any other kind of random collector can tell you there is a real sense of joy in completing a collection and having a huge inventory of things on hand even if others dismiss the collection as useless. Finding all kinds of things and keeping them, sorting them and eventually using them is wonderful fun - I love making a new character and finding all kinds of great stuff they can use hidden away in my stash.
The other big thing that ES does that seems great for a game long term is tiny improvements. Once you have an Harlequin Crest for Classic you never look at that piece of gear again. Nothing can improve on it. In ES that is almost never the case as you can collect jewels to crush together into better jewels which can crush together into even better ones which can be used in formulas to make even crazier pieces of gear. Every piece of gear can be tinkered to have more stats, forged to have more stats and dstoned to have more stats all in different ways and by collecting different things. That constant experience of finding simple things and having them slowly combine into improvements to your character makes for a feeling of accomplishment and whets your appetite for even more slaying and gathering. That combination of substantial upgrades that come periodically and small upgrades that come constantly is a recipe for addiction. Addiction, in this case, being good.