A few of the more problematic things at the top of my mind:
- Introduced cutscenes in dungeons. Because there are absolutely no problems to be foreseen there.
- Gated storylines behind early solo instances.
- Diadem. All of Diadem.
- Blocked /tells within instances.
And, next, an absurd thing:
- Introduced all manner of ridiculous flying mounts, while still arguing that certain mounts can't fly because they'd have to come up with an animation.
Oh, one more: a 1000-character limit on forum posts that can be evaded entirely by editing a post after the fact. Seriously, SE. What the hell.
