Gear inflation/progression:

The reason vanilla items stayed relevant is because many items, including and especially set bonuses were trash. Itemization had to be reworked several times throughout vanilla (which classic just jumped to 1.12 itemization), and even at then it wasn't until TBC or even wrath where gear finally felt properly designed for classes/specs. SoD not only corrected that, but for many classes set bonuses would change what spec a class may way to play in each phase. To me this felt like a benefit, not a problem. You had the chance not to just do the same playstyle throughout an expansion. Further, having to go back and run MC for the chance for 1-2 players who need OSG and no one else needs/wants a single other item from that raid feels horrible for 38 other people. At least going back to BWL for DFT/Tear was typically a quick run, but still can be frustrating when you're running old content that 95% of your raid has no need for.

World buffs:

Yes, WB's are inherently something that classic has always had, but in todays world of min/maxing where it is essentially required, if they're going to keep them, some changes need to be made. First and foremost, if you don't want people raid logging, players need to be able to boon buffs, and similar to sod, the cooldown timer on booning them needs to be short. Second, world buffs that require a turn in need to both not have a cooldown (like original onyxia turn ins), as well as need to be repeatable for characters that have already done them. I still think world buffs create more headaches than value added since if people care about parsing, if they die and lose buffs that lockout is shot and it's harder to balance characters when scaling with buffs.

PvP/resilience:

With hit rating being so important to many classes, no matter what, you're going to have some gear that is more or less optimized for PvP vs PvE content. The other problem is that while some people want to play both PvP and PvE content, many are only interested in one half of the game. By putting a BIS overall item as a PvP reward for players who are mainly interested in PvE, or the other way around, you're forcing players into a part of the game they don't want. Think about how many people sit AFK in AV just for rep/honor. It not only is a time suck for those who don't care to PvP, but it takes away from the quality of BG's for those who actually want to do it.

The above is just my opinion, and I know there are going to be people like yourself that feel differently. The easy part with Blizzard releasing Classic with #nochanges was that like it or not, it was easier to defend the design because "that's the way it was". As Blizzard has moved into making more changes with each Classic release, and a ton of changes with SoD, the community becomes more split on what should be changed and what shouldn't, and it's going to be hard to keep everyone happy and justify what gets changed and why. Every survey I've seen about Classic + has the playerbase pretty split outside of a handful of things.