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In Diablo II, there were some optional quests that would give you permanent stat boosts, such as the Prison of Ice quest that would give a permanent boost to your resistances.
Are there any quests in Diablo 3 that give permanent stat boosts?
No. There are not.
In fact, there are no 'optional quests' at all as it were. Most random world events, the closest equivalent, award an achievement and some loot.
Because you can reload and replay quests at your leisure there leaves little room to put in quests that have permenant stat rewards. With that being said, to this point there are no quests that offer a permenant stat boost.