
In Act V, The Siege on Harrogath will allow you to add a socket to any non-socketed item, and The Prison of Ice quest will grant you a passive +10 to all resistances bonus (+30 in total once completed at all difficulties).Waypoints Stat Boosts Zone levels Runes Mana Regen. In Act IV, The Hell's Forge quest will give you several high quality gems and a decent chance at a mid to high rune. In Act III, The Golden Bird, and Lam Esen's Tome quest are useful as they add 20 health points and 5 stat points respectively, each difficulty. These are of great help to all characters and should always be finished no matter what build or difficulty, for a total of 12 extra skill Den of Evil, Radament's Lair and The Fallen Angel all three give the player extra skill points when completed. There are several optional quests that players will probably wish to finish even though they are not mandatory. So while a player can technically run past The Summoner and just read the book, most will kill him while they are there. For example, The Summoner is technically not mandatory, but he appears right next to the book that needs to be read to complete the actual mandatory quest, The Arcane Sanctuary.
Some quests are also hard to miss completing. However if they player plays singleplayer, some become almost mandatory since they cannot be skipped by advanced players opening Town Portals ahead of the game (and they are a lot more appealing to do as you are faced with them anyway).
The quests listed above are required by the game code to be completed.
Act I and Act IV have no other mandatory quests. Rite of Passage must be finished before the player can enter Worldstone Keep in that difficulty. The Blackened Temple must be completed before the player can enter the Durance of Hate in that difficulty.
The Horadric Staff and The Arcane Sanctuary are both needed in order to reach Tal Rasha's Tomb where Duriel is. They can be seen as prereqs for the last quests of each act In these cases, these quest must be finished in order to reach the area the act boss is in. Some acts have other quests that are necessary to complete. Sisters to the Slaughter: Kill Andariel. Each act has at least one quest that must be completed in order to progress, and that is the boss kill quest that requires the player to kill a Great Evil.