Missed by previous replies I believe, your first legendary item only goes up to level 40, but tends to have good tiers/legacies, so don't rush to replace it without good reason.
Do not decon at 31 like someone previously suggested unless you are talking trash legendaries you've gotten later (51+) solely for the purpose of decon. The reason to decon at 31 is twofold, one, you get more relics than at 30, two, 30+ you get to select a legacy to turn into a scroll you may apply to a similarly leveled legendary. IE, once you know the legacies you like, you stuff scrolls in your vault. At level 60, find a legendary that has a couple already on it with decent tiers, apply scrolls with others, voilà, custom item with just what you want. (Note, legacies applied via scrolls go on at tier two.)
If you are grouping, you'll also want a buff stick (probably a moot point once Helm's Deep releases) which you swap to before applying buffs, then swap back to your regular item before combat. It helps if your buff stick is OBVIOUS compared to your regular item so you don't enter combat with it.
In Dolven View, there are repeatable quests to get legendary title scrolls for your items.
You want Ancient-dwarven damage type for your first few items, so you can get Thorin's rep out of Sarnur (MOBs below there aren't particularly affected by other damage types), and you can get physical mitigation from Midnight Raid in Eregion.
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