There are many many reasons why Diablo 3 is great, here some of them.
Note: This is an old article from 2008, I apologize for the old screenshots.
Graphics.
Let's start with the graphics. They have been upgraded to fully 3D from the older sprites. We now have a fast 3D rendering engine, that gives us great graphics, at great frames per second. With the new graphics engine, come great new visual effects. From the moves, to the enemies, everything looks much better, and more visually satisfying. Especially when you are hacking away at many many enemies, it is very gory, as we have seen from the gameplay footage.
Interactive Environments.
Diablo 3 features interactive environments that can either help or do harm to your character. In some parts of the game for example, you can use the environment to your advantage by smashing a part of the environment, and having the parts of it fall on all of the enemies standing directly next to it. Killing them instantly. Though in some parts, the environment can work against you. In one example we were shown, as many of you have seen, there will be chests that will instead of giving you loot, activate a device which makes enemies come out of nowhere, and attack you.
Amazing Skill Effects.
I know this should go into graphics, but it deserves it's own part. The visual effects on the skills are great. From what we have seen, it will be very satisfying using all of the skills, and we have only seen a few. There are still many more, as well as other classes to see. Each with their own unique look, this was a great move by Blizzard. Giving all of the skills great, unique effects. Sort of like in Diablo 2, but improved, a lot. Even the enemies use skills with amazing visuals.
The Improved User Interface
Diablo 2 had a great user interface, but Diablo 2 is also many years old. Blizzard took the original interface and upgraded the heck out of it. And I think they did a great job. It is just as before, when it comes to the health and mana bars, but a few things were changed to make it easier for you to play. As stated by the developers, the potions belt was replaced by a skill bar, allowing you to use your skills quickly and easily.
Customized Gear for Each Class
Each piece of gear is custom designed for each class. Barbarians put on demonic style garb, with devil horns. The other classes will have awesome gear that will make them stand out, and look unique.
Unique Looking Enemies..everywhere!
As you can see from our monsters page, the monsters in Diablo 3 are very unique looking. They range from ghost type creatures, to weird blobs, and even more bizarre creations, you won't ever get bored of slaying countless of these monsters.
Huge Bosses!
If you thought the bosses in Diablo 2 were big, you will love the bosses in Diablo 3. They are stated to be six or seven times larger than the player character. This makes all of the boss battles that much more epic, and challenging. As well as satisfying, when you finally beat them and visually.
Scripted and Random Events
As stated earlier, scripted events can occur at random so as you go through the game multiple times you don't see the same thing. This along with all the other random systems such as items, dungeons, maps, etc. make for a incredibly repayable game. If you thought Diablo 2 had replay value, you will love Diablo 3. They took the random system from Diablo 2 and improved a lot on it. Random items, random monsters, and random adventures. This allows Blizzard to place random scripted events almost anywhere, and it keeps the game fresh every time you play.
The Witch Doctor!
The Witch Doctor is a new class type in Diablo 3, it can cast out locus swarm, which spreads between targets and deals damage. It can also summon mongrel things that it can cast locust swarm on. Locust swarm buffs their damage. The Witch Doctor can also use a fire bomb spell. Can summon a wall of zombies that stays stationary, but the zombies in the wall flail their arms and do damage to their opponents. This has to be one of the coolest ideas for a skill, ever.
There are many other great things about Diablo 3, but I hope I covered at least some of the great ones. If you have some other things you would like to add, please post a comment. Thanks for reading!