Difficulty levels will be of great importance in terms of progression and re playability, just like in previous Diablo games. However, unlike previous Diablo games, Diablo 3 will have 4 modes as opposed to 3.
The Diablo 3 Difficulty Levels are:
- Normal
- Nightmare
- Hell
- Inferno (the new mode)
The new Inferno difficulty level, first announced in August 2011, will be aimed at level 60 players (the max level in Diablo 3). It will also feature:
- Monsters with a minimum level of 61
- Inferno-only item and rune drops that have unique models
- Monsters and bosses have more health, damage, damage resistance, aggro more, and have more powers.
Besides the Inferno-specific changes, Diablo 3 will also have more "general" changes that will make the game much harder than before. Diablo 3 will have less potions, less life leech, and generally less healing sources. Also monsters won't be covered in immunities and filled with buggy one-hit kills. In Diablo 3, the max level was 99 whereas the max level for monsters was 85, unlike the limits on Inferno.
The new Inferno Diablo 3 Difficulty Levels is planned to have a flat difficulty level, meaning that all areas from start to finish will be equally hard. You no longer need to run specific areas or do specific bosses for good loot like in Diablo 2.
It has also been stated that Nightmare and above difficulties will require more than one skill spam like the easier difficulty to beat, and players will be required to use tactics in order to defeat the monsters/bosses.
Multiplayer Diablo 3 Difficulty Levels difficulty will scale based on the number of players in the game and not their levels.
Here are 2 official quotes from Blizzard on the subject:
The general philosophy [of end game] is that we want to make finding better loot at the end game more enjoyable, and attempt to address 'path of least resistance' style gameplay.
There's a couple ways we're doing that, some of which we're not talking about just yet.
Primarily it's focused around Inferno and creating a consistent difficulty across the entire act. By having a flat difficulty across the entire game in Inferno it evens things out so that no matter where you want to go you're being challenged, and you have a good chance at getting good drops. That's intended to open the game up so there's simply more freedom for the player to go anywhere in the game and have an equal chance at good drops.
There will probably still be a slight difficulty curve in Inferno, as enemies simply become more complex and have greater abilities later in the game, but we're making an effort to make it as even across the board as possible. It's something we'll very likely have to keep tweaking after release.
In addition we're attempting to de-emphasize path of least resistance by creating a more equal value across monster types. We're not going to get into specifics about who can drop what or how much, but our intent is that being out in the world exploring should be as lucrative (potentially more) as simply aiming for the quickest-to-reach boss and running that over and over.
Getting people out into the world of the game and away from repetitive boss runs is essentially our goal. It's going to be tough because people will find a path of least resistance, and they will make that the game. We're going to have to keep on top of it, and truthfully it's something we're still working on. We'll likely want to have some additional features to support these ideals, but just getting Inferno tuned right is our current end-game goal for ship.
and
I don't think it's ultimately what we want for the end game either, but it's a great start, and we think plenty to keep people engaged for the initial release.
[...]
If we believe it will make their play experience more enjoyable or rewarding, absolutely.
The problem is people do boss runs over, and over, and over, for years, and it's not because the boss run is fun, it's because it's the path of least resistance to finding an awesome item. No one wants to do repetitive boss runs.
If we can instead say the path of least resistance is the entire game, that there's no single boss you have to repetitively farm to get the most items per hour, then the game is better for it and the players are liberated. They now have options to go anywhere they want to look for items. Hitting that perfect balance is probably near impossible, but we think we can get pretty close.