Ghostcrawler chimes in with Blizzard's analysis of how this happened in Wrath of the Lich King. He explains how PvE content was made very accessible, thus giving many players access to powerful gear (or weapons) while introductory PvP gear came much later and was difficult to get. This resulted in players entering Arenas with low life and no resilience -- you can probably imagine how that turned out. It didn't help that classes were dealing too much burst to begin with, which led to class changes through several patches. A final point he mentions is how BC granted a lot of Stamina that wasn't replicated in Wrath, which made players "feel a little fragile initially".