All the dungeons were linear and the introduction of heroics was the first step toward muti-level difficulties.
Stunherald and other RNG ruled PvP. Arenas introduction frankly ruined PvP shifting balance from group to 3v3. This began the homogenisation we see today.
Tier tokens
40man -> 10man -> 25man killed shit loads of guilds.
Introduction of flying mounts seriously damaged world pvp.
Daily quests became the standard for rep farming, not nessecary worse than Vanillas farm everything approach but a lot faster and led to daily hubs.
Pvp gear was from arena the only method of decent gear was playing in a sand pit fighting your own faction not the Alliance.
Druids in Arena