For me, the most stark change that patch 4.3 is bringing about is not a change to the systems or gameplay, but a philosophical change at the heart of World of Warcraft that spells out some of the potential big announcements that might be coming our way during BlizzCon.
Transmogrification is bigger as a philosophical leap than we think, and here's how.
The silhouette theory
World of Warcraft's factions are rooted in an idea called silhouette recognition. What this means is that you can easily tell who your friends or enemies are based on their aesthetic look. The reason there are no "humans" on the Horde side is because Blizzard wants you, as a Horde player, to point out a human on the battlefield. The same goes for every race. No two sides have races that feature identical silhouettes. This is even the reason worgen are forced into their bestial forms in combat -- players need to know that you're a worgen.
This goes for gear as well. Blizzard practically invented the gear tier system in raiding and made it so your two most prominent pieces, shoulders and headpieces, were the sought-after marks of power and prestige, and a physical reminder of player accomplishment. My shoulders currently tell you that I've been to the Firelands, killed many bosses, and taken their stuff. My silhouette is instantly recognizable as a raider who is geared and powerful.