Here a couple of things that come to mind for me:
1) Start developing separate raid content again. I dislike 10 & 25 man versions of the same raid. I dislike it immensely. No-one would complain about shared lockouts. They could produce much more content for people to get into it. They wouldn't have to endlessly re-balance the same encounters.
2) Bring back a real sense of progression - not just from a gear focus. I recall some of the best experiences I had was in BC with how well integrated quests, dungeons and raids were.