How do the different difficulty levels affect the gameplay?

Question

In Diablo III there are four different levels of difficulty:

• Normal
• Nightmare
• Hell
• Inferno

Obviously there will be more and stronger opponents, but apart from that: What exactly are the differences?

Answer 1

There are a few differences that are known and probably more as well:

1. Higher difficulties mean higher tier loot. Jay Wilson stated that over 70% of all of the armor items in the game cannot even be found in Normal difficulty. So as the difficulty raises so do the armor tiers. One can assume 4 per act totaling 16 tiers. Remember, tiers don't necessarily mean better, just different looking.

2. As the difficulty increases, the boss monsters become more powerful. More specifically, in each difficulty, the monsters gain one more special ability. By Inferno, they can have up to 4 special attributes which can make them very difficult to take down.

3. More obviously, the standard enemies will scale with the difficulty. This video says that monsters scale much like they do in diablo 2. They all have set stats which they carry with them as they level. The monsters also have more awareness and abilities as they scale through the difficulties.

4. Also, you progress through the difficulties, you can upgrade your artisans more through items that drop only in higher difficulties. However, as noted by @Kexlox you can also buy the tomes from the AH and upgrade your Artisans early.

Keep in mind while playing that Normal is made to basically be the tutorial and break you into the game. Experienced D2 players will cut through Normal with ease. As you progress, the game does get very difficult, and you'll be wearing your butt on your head once you reach Inferno. Inferno was basically built to murder you. Jay Wilson stated that they took the hardest difficulty that their testers could handle and agreed was fair and they doubled it.

FROM BRYSONIC'S ANSWER -

The difficulty also scales by having additional players affect monster HP. Monster damage used to scale, but no longer does as of patch 1.0.3.

Note: Because of patch 1.04 monster health per additional player now is not effected by difficulty.

Here is a table showing the amount of scaling that occurs based on the number of players in the game:

This table is now technically useless, but I will leave it here as a placeholder.

          Normal          Nightmare          Hell            Inferno
         ===============  ===============  ===============  ===============  
Players    HP    Damage     HP    Damage     HP    Damage     HP    Damage  
   1      100%    100%     100%    100%     100%    100%     100%    100% 
   2      170%    100%     170%    100%     170%    100%     170%    100% 
   3      240%    100%     240%    100%     240%    100%     240%    100%
   4      310%    100%     310%    100%     310%    100%     310%    100%

Answer 2

One important difference in difficulty levels that I haven't seen mentioned yet is the amount of monster scaling that occurs when additional players join the game. In normal, only monster health is increased as more players join, but in higher difficulties, both monster health and monster damage increase, and they increase at a greater rate than at lower difficulties. As of 1.03, only monster health scales with number of players.

Here is a table showing the amount of scaling that occurs based on players in the game:

           Normal     Nightmare    Hell      Inferno
         ==========  ===========  =======   ==========
Players      HP          HP         HP          HP      
   1        100%        100%       100%        100%     
   2        175%        175%       175%        175%    
   3        250%        250%       250%        250%    
   4        325%        325%       325%        325%    

Note: The damage increase per player has been removed in the 1.03 patch. The table has been updated to reflect this.

Note: Updated with 1.04 values. Source

Answer 3

Difficulties

Diablo has always been expanded through the use of difficulties. As you progress through the game and complete the story, new difficulties are unlocked allowing for more playable content.

• There are four difficulties; Normal, Nightmare, Hell, and Inferno.
• Players should be around level 30 when completing Normal.
• Players should be around level 50 when completing Nightmare.
• Players should be around level 60 when completing Hell.
• Each difficulty is unlocked by completing the previous difficulty.
• There will be no difficulty-specific content. Monsters change based on difficulty with increased damage, HP, defense, AI, and affixes.
• Some monster affixes only appear in higher difficulties.
• Higher level items can only be found in higher difficulties.
• There are a handful of unique monsters that only appear in certain difficulties.
• Elite monsters in Inferno will also have enrage counters which will increase the difficulty of the monster if not killed fast enough.
• Enraged monsters will range from added health, damage, increase in skill ability, and many other added challenges.

Inferno

• Inferno is a new difficulty added to the Diablo franchise through Diablo III.
• Inferno is the last difficulty for Diablo III.
• Players must be level 60 to access Inferno.
• Inferno is intended as the end game content for Diablo III.
• Inferno has been scaled way up with monsters showing increases in health and damage around 6 - 7 times that of Hell difficulty.
• After initial testing where testers felt Inferno was challenging enough, the doubled the difficulty.
• During the time of internal testing, not a single tester was able to clear Inferno.
• Monsters in Act 1 are level 61, in Act 2 - 62 and in Acts 3 and 4 - 63. And each of those have Tier items that ONLY they drop - so a total of 3 Tiers in Inferno.
• However, these tiers are primarily significant to the look of the armor.
• Along with more difficult monsters, Inferno will also have enrage counters on elite monsters.
• Enrage timers are an unseen amount of time to kill the monster before the monster gets an additional challenge to defeat them.
• Enraged monsters can gain things from added health, additional damage, and more powerful spells.

from: http://www.diablofans.com/topic/41359-recap-diablo-iii-difficulties/

Answer 4

To answer the question How is life steal affected by difficulty?

Per this post, Life Leech efficiency is scaled based on difficulty level

Life Leech gets passivly deminished every difficulty level to counter high damage builds which could achieve 9 or 12% ll with dps numbers close to 100k.

Life Leech Effectivity:
normal = 100%
nightmare = 70%
hell = 40%
inferno = 20%

Answer 5

The other answers here contain a lot of text but no short and to-the-point list, so here goes. Click the links for concrete data.

1. Monsters becomes stronger
   • Monsters have higher level, which means more HP, damage, armor and resistances.
   • Champion monster packs and unique monsters have more affixes and additional affix types are unlocked.
   • The bonus the monsters receive in multiplayer becomes stronger.
   • Crowd-control abilities have weaker effects.
   • Elite monsters get enrage timers.
2. Loot becomes better
   • Higher-level items drop.
   • Higher-quality gems drop.
   • There are more available affixes for items (e.g. elemental resistance affixes don't drop in normal, only in later difficulties).
   • Higher-tier crafting materials drop (e.g. Pages of Blacksmithing only drop in Nightmare, not in Normal).
   • Items salvage into higher-quality materials.
3. Your character receives penalties
   • The time it takes to swap a skill gets longer.
   • Your life steal is less effective.

Answer 6

The main difference is that monster level increases. A level 2 monster in Normal gets leveled up to a much more powerful level 32 monster in Nightmare. Higher leveled monsters have more HP and damage than their low-level brethren, and in addition, certain monster affixes (those titles that show up on rare and champion mobs) are level dependant. You will never see a pack of "Avenger" Champion mobs outside of Hell, for instance.

In addition, certain items only drop in certain difficulties - in addition to the level-based items (which obviously go up thanks to the increased monster level), non-leveled items like "Pages of Blacksmithing" will only appear on certain difficulties, and there is a new tier of crafting reagents as well.

Finally, on Inferno, bosses will have "Enrage" timers in addition to the level up bonuses. These are specific to Inferno and are essentially failsafes to prevent bosses being beaten by unexpected means. Kite the boss for too long, and he'll get a lot more aggressive.

Answer 7

The levels will also allow you to unlock certain ingredients, which you will require if you want to craft higher level items: