Melee vs Ranged
In Mythic+ dungeons melee do really well thanks to their cleave and mobility compared to casters that have to stand still.
Melee may be a little stronger in Mythic+ dungeons while ranged are a little stronger in raids, which is fine as long as the gap isn't too large.
The team doesn't want to solve the melee vs ranged problem by making life boring for melee and making raid mechanics that don't target them at all.
The community doesn't do a great job when something is 1% better and they claim it is the only viable option and everything else is garbage.
Ability Pruning
You prune a plant to guide its growth in new directions that are hopefully more aesthetically pleasing. It is removing stuff to make room to add new stuff.
One of the themes of Legion class design is to emphasize strengths and weaknesses of classes.
A lot of the things that were removed were removed after looking back at classes over the years where the class designers had taken the easy route. The team was coming up with a new set of abilities for an expansion or needed new talents and asked themselves what that class needed. If a class wasn't very mobile, they gave them a talent row with mobility options. Players thought this was amazing and what they always wanted, but that led to a lot of homogenization.
In a world where no one has weaknesses there are also no strengths.
In Legion there is now room to add interesting utility, especially for specs that feel like they don't bring much beyond their throughput.
Bring the player, not the class was a reaction to the hyper regimented raid structure of Burning Crusade where buffs were party based and synergy was more important than anything else.
The team would like to get back to a place where if you are a PuG leader, you are looking at your raid comp and saying, "We could really use a Hunter, let's look for a good hunter", rather than "Let's look for any ranged DPS, especially the flavor of the month that is 2% better and thus is the only viable one". Obviously this isn't the case literally right now, but it is what the team is thinking about and would like to move towards over the course of patches and future content.
Expansion Launches
There is a lot that has been done by the server team and how the world is designed that will make the Legion launch smoother than the Warlords launch.
Zones scale in level so the population will be distributed across the different zones.
Under heavy load there will also be multiple versions of the same zone on the same server to make sure that server performance is okay and gameplay feels good.
One of the things that caused problems for servers at Warlords launch, especially on the Horde side, was the Spyglass that was needed to start the Horde Garrison. It was only usable by one player at a time, which caused a bit of a bottleneck.
The team hotfixed the Spyglass to be usable by multiple players at a time, but something went wrong with the hofix and then no one could use the Spyglass!
Horde players in North America were all stuck there for a few hours until the team could fix it, resulting in a big clump of players all hitting the garrison at the same time, making server problems much worse.
Everyone on the server was doing Frostfire at the same time, resulting in a laggy zone and even more players building up there, making things even worse.
Raid Structure and Rewards
The current raid structure didn't change this expansion because there isn't a huge problem to fix. All of the raid structure changes in previous expansions were targeted at solving a particular problem.
Classic to Burning Crusade changed because the logistics of 40 man raiding were overwhelming for a lot of guilds, players were a cog in the giant raid machine. "Your job is to dispel this one person and that is your only job in this giant 40 player group". It was fun for the raid leader to plan all of that out but less fun for the players.
The introduction of 10 player raiding allowed the team to let more types of players and groups experience the content.
Flexible raiding is probably the best raid structure the team has done and is working well.
Three difficulties plus Raid Finder means that there is a raid difficulty for all of the different groups. Normal for friends, family, and PuGs. Heroic for guilds that are formed for the purpose of raiding, Mythic for the extremely hardcore players.
Mythic needed a fixed size so that tuning could be done in a way that is satisfying.
Being able to split the raid into groups that could go off and do a mechanic and assuming that the raid had one of every class was part of the encounters that the team wanted to design for Mythic, so the 20 player size felt like the right number. A smaller group size such as 15 players might have also worked, but that would have been tougher for the groups that were already at 25.
The team was worried that LFR was growing too much in prominence in some ways, so they wanted to make sure that players were doing regular group raiding didn't also feel obligated to do lots of Raid Finder to complement that. The team probably overdid this in Warlords by removing Tier sets from Raid Finder, so they are coming back to Raid Finder in Legion.
Most of the players that are raiding competitively will already have set pieces before that Raid Finder wing opens, so they won't feel obligated to go and do it when it opens.
If you have been really unlucky and want your set bonus, this gives you another chance to go and get that last piece.