Here's why I lost interest in WoW:
I don't like to think that I'm one of those people who claims that vanilla or old WoW is better because honestly, I do think the game has increased in quality over the years.
My primary issue with WoW is that in the old game you had a lot of complexity in pvp in the form of damage resistances and the talent trees offered a great degree of control over small stats. A lot of people justified the changes to the system by basically saying that everyone was running cookie cutter setups from websites that have been mathed out, but I felt that just wasn't true. I believe it's true for raiding where someone has figured out how to do max dps with minimal threat but this completely ignored the battleground and world pvp aspects of WoW.
For example, me and my friends in vanilla experimented with a lot of different gear setups to the point where we had different gears for different damage resistances and then we would tailor our classes to be very good at specific things. For example, we once did warlocks for maximum damage and critical chance such that 4 of us could alpha any other class with a 4 sec cast spell.
Then another setup was maximum tank with palladins with each of us carrying gear that was setup for resisting different classes with one "omni-tank" gear set.
The idea is that people who go solo tend to just focus on buffer (stamina), but if you got 2 people to focus on resisting and reducing damage, then you could start to take on greater numbers as people would start to struggle to kill you.
Nowadays this is impossible. You select a talent spec, it gives you skills automatically and your stats come purely form gear.
This alludes to the last point which is that the over-emphasised arena pvp because now that they have classes simplified, they now focus on balancing classes with trinkets, cooldowns and abilities to make a "meta game" in arena. But this ultimately left battlegrounds into nothing more than a zerg war rather than a place where you could try to employ more refined tactics.