The most appealing part of this game for me is the smooth transition between action and strategy. You can be running down a hallway and mowing down goblins, then suddenly you're inches away from a giant, sifting through your inventory and trying to decide which item is the most likely to prevent his club from crushing your skull. Do you have a sleeping potion to throw at him? Maybe this unidentified scroll can teleport you to safety? Your wand of polymorphing might turn him into something that can't wield a club...
Looting is another big part of the game that stands out for me. Almost every enemy drops something, often many somethings. My inner hoarder can't help but squeal in delight when I've finished slaughtering a battallion of gnomes and find myself standing knee-deep in daggers, various wands, and a pile of tiny outfits. You have a fair bit of inventory space, and it's surprising how fast this can fill up with scrolls, potions, books, gems, weapons, rings/amulets, armors, foods, etc. Then you have to decide what you want to identify, sell, drink, read, or keep for emergencies.
Don't get me wrong though, having hundreds of items and unlimited time to think doesn't make the game get any easier as you go further down. Enemies become stronger and spawn more quickly, until you're fighting your way through waves of liches, giants, skeletons and demons. Your stockpile of healing items, wands and scrolls can be quickly depleted if you don't plan ahead or if you just have ♥♥♥♥♥♥ luck. Beating the bosses and making it back to the top is a major accomplishment, one that you'll only experience after you have a good grasp on how things work and what the best ways to approach situations are. Even then it's still a rather rare experience, especially with the most difficult characters.
There are only a few games that I can say I've put hundreds of hours into, and I'm proud to say that this is one of them.
TL;DR - Replayability, looting, inventory-management, strategy.