Professions and crafting are something the team will be updating in both big and smaller patches over the course of the expansion.
The intent is for professions and crafting to be parallel to the available content.
In the future, the team may use the world quest system in other spaces and have more of the world be a playground, but for Legion the focus is on the Broken Isles.
In an upcoming build, World Quest rewards will start at item level 805 rather than 810. Rewards were a little too powerful compared to dungeons.
Gear and Dungeons
You can get gear with an item level that is competitive with the very best Mythic raid loot from dungeons, but there are some specific trinkets, set bonuses, and other things that you will have to go into raids to get.
The Challenge Mode model of scaling down your item level, disabling set bonuses, and disabling raid trinkets effects was one option that could have been used for Mythic+ dungeons, but it felt like you were nerfed as soon as you walked in.
Melee and Ranged
The team keeps in mind the ranged vs melee advantages when they are designing fights.
Most players have a preferred play style, either melee or ranged. You don't see a lot of players that do both.
The melee loving population will probably be divided between more classes and specs now, rather than mages suddenly deciding to play Demon Hunter.
Melee rosters will probably shift around a little bit, but the melee and ranged composition of raid groups isn't likely to change.
When you hit Level 102, there is a quest in your Order Hall that allows you to pick a second Artifact weapon if you want.
It is intended that the best way to use Artifact Power is to spend it as soon as you get it, rather than holding on to it.
There is still room for improvement as far as having both tanks actively doing something rather than swapping back and forth.
There is a PvP mechanic that lets tanks safeguard another player and share a portion of their damage. The team has looked at bringing that mechanic into raiding.
In the long ago past, the team just made fights that needed a specific number of tanks. Tank swapping is one solution so that all of your tanks have a role in an encounter.
The long term goal is to make tanking engaging for all of the tanks, giving them tank things to do beyond a DPS rotation on the boss.
Talent and Spec Swapping
The change to talent swapping comes down to which situations the team wants you to be able to respec in. Lots of them to consider.
Should players be able to respec for a rare encounter out in the world
Should players be able to respec for trash packs and bosses in the dungeon
In battlegrounds, you are forced to play one spec for the entire game, rather than changing your spec to run the flag and then another for defense. The talent and spec choices need to be meaningful.
The talent changing consumable is pretty similar to the food and flasks that raids have been using for years.
The final system gives a nice default experience for a player. You can change your talents in town. There isn't any need to make or buy a reagent from a vendor or carry it around for most players.
Legion already makes changing specs easier and more flexible.
You can change to any spec, your action bars are remembered between specs, you can change talents in town for free, the UI has been improved.