If you already have your legendary weapon, or don’t feel like grabbing it yet, there is still an absurd amount of stuff to do. You could polish off Shadows of Angmar and finish Epic Volume 1 with the new Book 15. Heck, you can even roll yourself one of the new Classes; Warden and Rune-Keeper, and start from square one. I haven’t tried them myself yet, but they sound quite interesting, and people who have played them sound pretty excited.
Most noticeably you will spend some time taking advantage of inventory improvements. Any quest item that you don’t actually need to use for the quest (like a letter you need to right click to read) will go from your inventory to the quest log itself. You’ll still need an empty spot in your inventory to pick up the item, but it then zaps right to your log.
A “key-ring” system means that any keys that are permanent (as opposed to single use, e.g..: the Carn Dum ones as opposed to the Barad Guralan summoning ones) can be removed from inventory by right clicking. They will now appear as skills in your character window. Many inventory icons have been updated and are much easier to identify at a glance.
You’ll need the improved icons and extra space for crafting. A new 6th tier has been added to all of the crafting skills, and Crafting Guilds have been introduced. Like reputation factions, Crafting Guilds offer extra recipes and discounted supplies to their members. Note that, unlike with reputation, you can only be in one Crafting Guild at once.