TLDR.
1. Throughput Legendaries
2. Titanforging
Throughput Legendaries
In my opinion this is the worst idea, and most toxic system that has ever existed in the history of World of Warcraft. There are a few main points that make it absolutely miserable.
1. There are simply just way too fucking many in the game, and if you're playing casually it still takes about 3-4 weeks to get one.
2. This makes it almost impossible to get legendaries across multiple specs
a) This is even worse on alts or players who are trying to return to the game
3. It locks way too much gear. Between legendaries and some classes being forced to wear 6 tier pieces, that's half of your character that is locked to a specific item.
a)This automatically invalidates almost half of the gear in the entire game, combined with a good portion of the gear being bad in the remaining slots because of how imbalanced secondary stats are for many specs
b) SO MUCH LOOT IS WASTED/DISENCHANTED, it FeelsRealBadMan
4. Many of the utility legendaries are actually pretty cool and/or fun, but no one will ever use those over a 5-8% dps increase provided by a throughput legendary. It's very simple, no one will ever do that in any raiding setting. Not once, not ever. Prydaz is the only exception in high end progression cases.
Titanforging
This one is pretty simple. I like the idea of the system for the most part, but like many things in Legion it just takes the RNG way too damn far. This is perhaps more personal so I will just list the two things that I think would make the system much more enjoyable for me. Instead of it feeling like you can get upgrades, even as a mythic raider, by helping your friends or just doing old content. It's more devolved into feeling like
YOU'RE FORCED to do everything, because the Titanforging doesn't seem uncommon.
1. Titanforging is capped at the base mythic item level, 930 in the example of ToS
a) This is still A TON of room for players to be rewarded, even your mythic player has those couple of slots they could get, but it greatly reduces the need to constantly do everything for upgrades
2. Mythic loot can warforge at +5/10, and Mythic bosses (only Mythic bosses) drop a currency similar to old badges that will also allow you to upgrade to that cap if you don't get lucky. Let your +15 weekly M+ item also fall in this category (or whatever the max level reward is at the time)
a) Mythic raiding needs to be more rewarding for the effort than it currently is. I think I can speak for most mythic players in that not being able to combat the loot RNG in Legion at all is VERY frustrating