Latency issues are a big one. I don't know about lately but server was crashing more than daily.
Gathering issues. I've heard one or two people claim gathering is fine, but most think its really bad, and I haven't heard it's fixed. It's why I've quit.
Lack of a test server is a big one. Can't easily test builds so people get bored of grinding for the next build and quit. I wanted to start a site devoted to builds and I know TommyRuin loved doing build videos, but without a cheap way to test builds the theorycrafting ccommunity is thin.
Speaking of thin community: one-sided battlegrounds and a hard time finding dungeon or raid groups for your level, especially for a US player, makes it feel like a single player game.
There are issues with spells or talents not working.
The developers don't seem interested in addressing most of these concerns.
I do think talents and REs need to be revamped completely, but as I said in your previous thread: I dont want prereqs. I dont want to be forced to get X amount of fire spells to unlock the fire spell I want, because then everyone else will also have those fire spells.
So, to sum up:
A talent rework is needed. It is not even top 5 on my list of priorities, and tiers or prereqs are not the answer.