- They will be rare. Not every character who can use a legendary will get one.
- They will be very powerful. A legendary is not expected to be replaced until the next expansion, while most weapons are expected to be replaced in the next tier.
- They will have a significant quest or lore attached to them.
In the past, these three elements have caused issues with each other. Rare and very powerful means that guilds who can recruit multiple legendaries have a significant advantage. Conversely, guilds who lose their legendary feel resentful and unhappy. There is also significant conflict over who gets the legendary first, because the people later in line may not get one at all.
Legendaries being Best-In-Slot for an entire expansion also means that they can only show up in the last or second last tier. As well, classes which don't receive legendaries do feel slighted.
The quest attached causes three problems. First, only a small fraction of players will see the quest. Not all rogues will see the daggers quest line. It's restricted to raiding rogues who get the legendary. That seems like a very large set of restrictions.
The second problem is that the questline is a very long grind, because that is how Blizzard limits the supply of legendaries. I suppose it's better than the random chance method that previous legendaries used.
The third problem is that classes and specs which are not in line to get a legendary will not get an epic quest.
I like the quests and lore attached to these weapons. But I really wonder if the first two points are worthwhile. On balance I think those two points have caused more trouble than they are worth.
Can we have weapons with quests and lore without having them be very rare and very powerful?
I think we can. There is precedent here: [Benediction] and [Rhok'delar]. Both those weapons are fondly remembered by their respective classes because of the quest chains attached to them. I'd wager than the vast majority of characters who got those weapons back in the day still have them banked.
Both those weapons were good weapons, best for that tier. But they were still replaced in the next tier. If all your priests had Benediction, it didn't unbalance the raid. They weren't super rare, but were still worthwhile to get.
I think [Benediction] and [Rhok'delar] are the better model for quest and lore-driven loot. Solid, well-loved weapons, with interesting questlines, but without the unbalancing effects of rarity and power that current Legendaries have.
It would be easier for Blizzard to add more of these, even for more obscure classes and specs. For example, [Fandral's Flamescythe] might have been a very nice questline weapon, even if it isn't as powerful as a Legendary.
In my view, legendary weapons cause more trouble than they are worth. It seems to me that we can get almost as much mileage out good epic-level quest-driven weapons modeled after [Benediction], without the drama and unbalancing effects of legendaries.