All of my friends who bought Legion, all of them, are railing on it very, very hard and I'm reconsidering playing.
1. PvP gear has no progression. Before you could stack stamina as a healer since a lot of healing went to overhealing. You could use custom gems. Different trinkets, and so forth. It didn't take long to get a base set of honor gear either, so people saying "people in greens should automatically compete!" is ridiculous. So now everyone is playing the EXACT same character as everyone else with small adjustments either up or down based on iLvL, which removes the RPG aspects. There's absolutely no customization, it's just bad design. All so that they could concentrate on balance yet there is no...
2. Balance. I hear it's terrible. People have said that DPS can outdmg your heals and without interrupts 100-0 a healer in less than 20 seconds. That's unacceptable. Considering they gutted PvP gear and removed it from the game for the sake of balance, the current state of fears just doesn't cut it.
3. Streamlining. Again, removing the RPG aspects of an MMORPG is never good.
4. Ghostcrawler gone. The current dev teams goal is to obviously cater to the 97% instead of Ghostcrawler's philosophy of making content more difficult to encourage players to beat it, thus rewarding them. The best expansions were TBC/WotLK with these mechanics. The worst? MoP, WoD, and probably Legion. The current dev team doesn't have any fucking clue what they are doing.
5. LFR. Ghostcrawler said this was his worst addition to the game and regretted it immensely as it took out the reward of finding a guild or a group of players to play the game with. Let's face it, raiding was never HARD, it was just effort of finding a guild.
If they're right then this is quite possibly the worst expansion ever released and the game will tank extremely fast once the polish wears off.
It was built for the casuals, with everyone having the same character (with a different gimmick weapon), yet is run by a dev team with drastically untraditional philosophical goals, and they're likely to shit the bed...again. It's just a repeat of WoD, except worse, except a higher budget and better production values. If they don't do a really, really big balance pass soon...