Firstly, if you want to be a purist, you can always play the game as it is. Here you will find many tips, simply choose what you want and maximize you fun with D2.
1)increasing game speed.(Single player/Lan games,works best single player)
I made this post before, many people got upset by the word "cheat". Simply put, using this method you can increase game speed. Adding a 10% increase you can win dozens of hours with virtually no down side. Depending on your class and game style, you can choose the best number for you( from a perfect 30% to an insane 500% for farming depending on the circumstances). This will make the game a lot less tedious.
2)Plugy (Single player/Lan games)
Like the first, this doesn't affect game play in any way(except inventory charms, you can simply limit them to keep things fair). With this, you will have a bigger inventory,infinite stash and gold size , also shared stash with your other characters. Diablo 2 has some realm ( online ) only features like runewords(an item type) and special bosses, this activates those. Relocating skill and stat points is also possible.
3)Diablo 2 multires (Single player/Lan games/custom servers)
This is a simple mod, it increased the resolution of the game, you have a lot of options. D2 runs at 800/600 , it can feel very claustrophobic. Even running it at 1024/768 only will make the game more enjoyable without messing with balance too much. Higher resolutions give a huge advantage to some classes like Amazon, because you can hit enemies from very far away. However using melee classes won't mess with the game too much. Installation is more complicated however, you will need an older version of Diablo 2 to make it work. For that, you need either a D2 disc , downgrade your installation(not recommended) or a pirated copy. As far as I know, the blizzard installer doesn't let you play older versions.
Website: http://www.medianxl.com/t1716-guide-to-multi-res-w-plugy-for-diablo-ii(ignore the part about Median)
4)D2SE mod manager(includes plugy) ( Single player/Lan games/custom servers ( maybe works on the official too, didn't try))
Diablo 2 has many mods worth playing, you spend dozens,hundreds of hours playing different kind of mods. In the old days you would need have multiple copies of the game to make it work, now it's easier. With this program you can have as many mods (and play older versions of the game) as you want.
Website: http://snej.org/forum/index.php?PHPSESSID=b6310660d1667101199ef0ddc9cb21a1&topic=18914.0 ( instalation + some mod list. also check out http://www.moddb.com/games/diablo-2-lod/mods if you wish)
5)Diablo 2 reworked:
This is essentially a more casual/fun mod for D2. Everything is faster and more plentiful. Higher drop rates, higher Xp gain and so on. The core of the game is the same.
Hope this helps. I will make more edits if anyone has suggestions.
EDIT: bonus screenshot. Plugy also shows different statistics like this
EDIT 2: full features for PLUGY: http://diablo.gamepedia.com/PlugY