I have been playing this game since vanilla, and in my opinion, most "core" changes blizzard has done to the game (mostly in the name of "accessibility" and "convenience") has made the game worse.
The Burning Crusade:
The concept of expansions nullifying all previous gear progression
The whole consept of expansions making all the gear progression worthless. It makes gearing up towards the end of an expansion seem like a waste of time. Gearing up used to be seen as an "investment". With Blizzards "yearly expansions" this is a bigger problem than it has ever been. I don't have a good solution to this, I am merely pointing out the problem.
The introduction of flying mounts
We all know the arguments for and against this. In my opinion, this should never have been introduced at all. IMO, flying mounts + easy leveling = no World PvP.
The introduction of heroic dungeons
Dungeons should have 1 difficulty. Some should be easy, some should be hard ("heroic"). It is just lazy to upscale health and damage to make dungeons harder. Having only 1 difficulty makes people want to gear up so they can experience the hard dungeons.
The removal of heroic keys
Even though the concept of heroic dungeons is flawd to begin with, the farming normal dungeons in order to get "keys" to heroic dungeons was a good way to feel progression, even when only doing dungeons.
The removal of attunements
At some point during the expansion, Blizzard decided it was too much of a pain in the !@# to get attuned to Black Temple, so suddenly people could just go there without getting attuned. Why? These attunements was a great way to feel progression.
The removal of the "honorable kills" system
The concept of farming to get new ranks was so interesting. Setting rank requirements to acquire PvP gear as a carrot on a stick. Getting a new rank was just the best feeling in the world, and a huge status symbol. Now, we get "Honor points" and "Conquest points". How incredibly uninspired and no way of feeling progression (gain points, spend points, back to square 1).
Wrath of the Lich King:
Cross realm dungeon finder
Just queue up, get teleported to the dungeon, never talk to anyone, clear the dungeon, ninja some loot, leave and never see them again. What happened to the RPG element? Server community? This was the start of the complete ruination that has happened to these elements.
All dungeons are now easy
This was the expansion where dungeons (both normal and heroic) got incredibly easy. People started pulling 3 groups at once to effectivize the clearing. No challenge = no fun.
"Everyone should be able to see all raids"
They remade Naxxramas into a walk in the park. They introduced the "normal" and "heroic" versions of all raids, so more people could see them. Raids should only have 1 difficulty, and people should have to gear up and put in the effort to be able to clear them. If I say "remember how hard Illidan was?" everyone that did him, will know exactly what it took to defeat him. If I say "remember how hard Arthas was?" people will ask "normal or heroic?"... There is little insentive to bother doing the heroic version of these raids. Slightly higher ilvl and a different color on tier sets? Come on...
Both 10 man and 25 man versions of raids
Both dungeons gives the same loot. Why bother gathering 25 people when you can do it with 10? This expansion ruined sooo many guilds, including mine... 10 people is way too little (imo). Raids should have 1 size. In TBC they made some instances 10 man and some 25 man, a way better and less lazy system. I totaly lost interest in raiding seriously from this expansion and this change.
The renerf of dungeons
Blizzard finaly made dungeons hard again, but only 1-2 months after release, they nerfed them hard again... See the above reasoning why dungeons should be hard.
Leveling is now easy
I can't remember if it was Cataclysm or WotLK that made leveling incredibly easy and a walk in the park. By making leveling that easy, getting gear is no longer a fun part of leveling (it is irrelevant), making doing dungeons less attractive. No longer was there any point in getting a group for hard quests. Quests now came with ingame makers on exactly what to do and where to go to complete the quests. Leveling a character to max level used to be an achievement and a huge (fun) part of the game, now it was completely boring (no challenge).
If what you care about is defeating all bosses in the game and see all raids in the game, and this is your motivation, then you are out of luck from this point. Now you just queue up LFR, clear the raid in 30min, now you are done. No challenge, no fun.
Mists of Pandaria:
The removal of talent trees
Fiddeling around with talent trees was so much fun. Sure, it had been dumbed down in both WotLK and Cata, but at least it was still a part of the game. Sure, "everyone had the same talent tree", but that did not stop it from being interesting. The new talent tree is still one where everyone takes the same talent...
The removal of rating requirement for PvP gear
Gearing up in PvP from this point was has just been "quque up 2v2 every week untill you have all the gear, then you are done". Gear progression is what is fun, not achieving a high PvP rating or whatever...
Warlords of Draenor:
Made the classes lose all complexity and uniqueness. Luckily for me, I play Enhancement Shaman, and they forgot to prune this spec so that is still fun.
Professions are now irrelevant
Leveling up professions was a fun part of the game, and something we could do to optimize our characters. I just don't see any reason to make it irrelevant...
I am not going to even bother mentioning Garrison. We all know it was a huge mistake.
All of these changes has been changes to make the game more "convenient". More "accessible". But in the progress, it has turned the game into a lazy game without any sense of progress or accomplishment.
Do you agree? Disagree? Why?
Can you come up with other examples?