The worst thing Blizzard ever done imo was redesigning 1-60 content. What I'm pointing at in particular is the redesign of many of the zones.
Making them 100% linear was a big mistake. Took away the feeling you were in a world and there was no point in exploring, since nothing was off the one golden path.
I liked Wrath the best. Although TBC's Kaza is my favorite dungeon/raid and made shadow priests a viable spec.