I’m so glad you asked. There’s tons and tons of little things that change when dual spec is added to the game. It’s hard to quantify them all. The best way to put it is it makes players think about their characters very differently. A character who can change between a PvP and PvE spec at will is inherently a LOT more powerful. All of this changes the way the game feels, and thus removes a LOT (and for me, ALL) of the classic energy
I will not, EVER, play a version of WoW that has dual spec.
Here’s an incomplete list of specific ways dual spec hurts the game: