Many of the areas in the first 5 main worlds of the game can be summed up as maze-like configurations of hallways with an occasional landscape feature or NPC in them. Every other world in the game has varied landscapes and makes each area feel unique, and really makes the first 5 feel like a drag in comparison, going through what seems like the same area over and over again, just with a different name.
Even in different worlds, the layout is nearly the same, hallways after hallways, just with a different skin. Just take a look at the maps of areas in the first 5 worlds, and compare them to maps of areas in other worlds. The difference is huge, and while the map design isn't the main focus of the game, it's still a big part in immersion and enjoyment of the world.
Each time I make a new wizard, I dread having to go through the first 5 worlds again, hallway after hallway for hours or days. I would rather KingsIsle take the time to redesign these worlds than put out an entirely new one for their next big update, because I find it to be a major problem that can be off-putting to new players and re-players alike.
Personally, I'm most looking forward to what they'll be doing with the skies. What if they add in an endless changing sky that changes from night to day? I really love the pinkish clouds that they have in Pirate101's Mooshu. It's so pretty!