Hmm.. well, I have to say that I really dislike the direction they have gone with the Diablo series... in the first Diablo game, the combat was a lot more one-on-one with your enemies, and the creatures usually put up a much better, and more engaging, fight. I think the biggest element that separates the first from the others - is the darkness. It was often very hard to see what was coming for you, out of the shadows. But then the world became bright and shiny and the monsters weak and pitiful in Diablo 2 and Diablo 3. Notice in the
gameplay video for Diablo 3 he drops blocks on the monsters, and throws them off ledges, making for an easy fight, with no afterthought or memorability at all.
I would say that the pace is a very big factor in this. The speed of the fights are significantly different between the games. The items, massive stat bonuses, and all the new abilities that allow you to attack large areas of monsters, break that illusion of strength that appeared in the first, making the value of the combat seriously diminished. In this last trailer you can see the barbarian practically wipes his ass with the monsters in that dungeon. It takes only a few moments to kill about 50 goblin creatures... where is the fun in that?
It used to feel like an accomplishment when you killed something in Diablo, monsters were very frightening and would often make you jump. I recall fighting the Skeleton king Leoric, which scared the shit out of me. I ran through a door figuring he was way too tall to fit through it, but he ducked down and continued to chase me down! I was excited when I finally killed him after dying several times, it really felt like I had done something.
It's amazing how the items have really affected the gameplay, it used to be all about the dungeon crawling, the immersion, and the fear element, but it was something when you found a unique item in Diablo. In Diablo 2, unique items were literally EVERYWHERE, just about every level 99 had 2 stones of jordan, spiderweb sash belt, some amazing amulet that gave +3 to all skills, Runeworded itemes up the ass, and a complete other set of items that gave them Magic find so that they could get more shit which they could trade to get even more shit. In Diablo 2 the focus turned to items, the game's main objective became GREED. This really killed the rush of finding a Unique item, most of the time when you found a unique item, it was only junk, unless it was that certain Unique item you were looking for.
In Diablo the environment felt hostile, like there was a horrible past to every room you entered. The extreme darkness of the places would fill you with doubt and insecurity when venturing into the dungeons. It also felt VERY BIG, without waypoints it could take a long time to get down to the end making the cathedral feel like a vast place. In Diablo 2 it was all about RUSH RUSH RUSH, people would make a town portal to all the end bosses and you could beat the game(on Hell difficulty even, with the exploit of a glitch) at level 1, but only with the help of a higher level friend.
The distinct separation of character classes was another thing... No longer could a warrior class cast the same spells as a magic one.. or could a magic class pick up a claymore and use it in the same way as a warrior class did. There was a distinct difference in all the classes which obviously made some much better than others. For example, if you wanted to do "Uber Tristram" in Diablo 2 you would discover that the primary character in Uber Tristram was the Paladin. The Paladin used ONE skill and put most of his points into ONE stat. He maxed his holy hammer abilities and the synergy abilities that made that ability do more damage, and put all his stat points into mostly vitality with some strength(so that he could use armors). This made him not only an unstoppable juggernaut, with thousands of health, but the most powerful character in the game, dealing out tens of thousands of damage with his hammers.
Diablo 1 was a great game, which deserves to be remembered. Though I will buy Diablo 3, I still hope that some day an ambitious team of developers will create a game that recaptures that which made Diablo such a great game.