A lot of game content in Diablo 3 is determined randomly. Arguably, more randomness means more diverse content and therefore players less likely to become bored by repetition. But randomness also has drawbacks.
Maps are generated somewhat procedurally. Some elements like the map borders remain constant, but events and dungeon entrances are picked randomly among a pool and placed randomly in predetermined places.
Maps like The Den of the Fallen sometimes have loops, which is annoying: players kill monsters on their way, but when they realize they have been looping around, they go back on their way without any monsters to kill. It is a waste of time. A solution could involve using heuristics to prevent loops.
A Greater Rift (GR) is a dungeon in which players race against time. The higher the level of a GR, the tougher its monsters. GR is a competitive activity: leaderboards show the teams who achieved the highest GR levels. Problem: the GR map is picked randomly; sometimes it is a maze, sometimes it is linear, and sometimes it has loops. The randomness does not make for a fair competition. One solution would be to remove maze maps from the selection. Another solution would involve keeping the map and monsters constant, and make one leaderboard per map. It would be pretty much like a racing game.
Elite monsters can have up to 5 affixes. Affixes have 3 categories: crowd-control such as Vortex, defensive such as Extra health, or offensive such as Desecrator. These affixes are picked randomly, with a couple heuristics making sure that an elite can have at most one crowd-control affix and at most one defensive affix. Some affixes such as Invulnerable Minions were so annoying that they were removed from the game altogether.
Still, some combinations remain more deadly than others. For example, elites combining Molten (they burn where they walk on), Fast, and Horde (create 8-10 Molten and Fast minions) are the bane of melee characters.
In the original Diablo 3 (also known as D3v for Diablo 3 vanilla), loots were completely random. For example, a Wizard could drop a mace only Barbarians can use. Enters the Auction House (AH for short): the Wizard sells the mace on the AH and, with the money received for it, buys a better wand from the AH. Trading through the AH worked well from launch in May 2012 until August 2012 when the designers realized that the AH
can short circuit the natural pace of item drops, making the game feel less rewarding for some players. Indeed, players could obtain second-best gear in only a couple days, play for a week, get bored for not progressing, and leave the game. Conclusion: completely-random loot with an AH for trading does not work.
Loot 2.0 landed in the game just before the Reaper of Souls expansion. Loot 2.0 has 3 parts: 1) little to no exchange between players (to prevent gold farming), and no AH, 2) loot is targeted to the player, ie Wizards only drop Wizard gear, and 3) the Mystic lets the player reroll an attribute on any piece of gear. Six months after the release of the expansion, this has been holding fine.