Well, I absolutely and utterly detest Diablo 2. Yet, even with its similar combat system, I find DD to be highly enjoyable. (And I'm quite hard to please.)
For me, it's a combination of various factors. The attention to details, such as tributes to Polgara and Drizzt, along with the little odd bits like the conversation between skeletons in the first dungeon or the wishing well, appeals to me. The combat is actually fun, especially playing a mage (and I was playing one in my first time through). The first time is really the best: each map is quite large, and gives a feeling that the world is a big place (although it isn't that big; nothing the likes of Morrowind, but it certainly is a lot easier to navigate!). Unlike many other generic, vanilla RPGs, some characters in DD are memorable. (I find a certain orc particularly... cute. Ahem.)
The demo is a poor representation of the game, IMO. You don't really get to do many sub-quests and explore.
