What's the one thing Blizzard would have done differently in Cataclysm?
We would have worked the story around a continent rather than having zones. There was too much teleporting, and the sense of world was eroded. There was a lot of value in coming over the hill and seeing into the next zone, the transition. The teleporting created an instanced feel, and there wasn't the novelty. Mists has a whole new continent to explore, so that's something Blizzard have changed up for this expansion.
Why did Blizzard go with Cross-Realm zones over server merges?
Cross-Realm Zones were designed to fix the issue of having a ton of players at max-level, but lower level zones being dead. The experience of being in a MMO is lessened, so we wanted to fix that.
Closing and opening realms all the time is not the solution, we can't keep doing that incessantly. We also don't want 20,000 players running around Stormwind or other high-level zones! For players who like deserted zones, we're not really catering to that. WoW is not set up to be a single player game!