All of the RPG elements of actually being a Rogue. I really feel the class has been diluted massively.
I used to LOVE Rogues when I started in Vanilla. Despite the massive inconvenience of it, I didn't realise how much I'd miss things like mixing up my own poisons and filling my bags with them, poison and class trainers hiding in the basements of buildings, stocking up on flash powder, carrying around my lock picking tools, levelling up lock picking as a skill... all of these things made a rogue FEEL like a Rogue.
Now from an RPG perspective we're just a pretty generic DPS class.
(also I miss Pandaria Burst of Speed, it was so useful for getting around old dungeons)