New Leaf is the first Animal Crossing yet that feels like a proper sequel rather than the same game. Sure, all of them have had new buildings to shop at, new bugs/fish to catch, and new ways to play with each other online, but New Leaf is the first to have an entirely new mechanic in becoming the Mayor. This also allows your entire town to be improved at your own leisure.
There's also tons of new customizable opportunities (upholstering furniture, loads of new clothes and shiz, etc) for your items, and your house is now basically fully customizable from the outside as well as the inside now. As the mayor, you can build new buildings/bridges/other goodies for the town. It really creates a nice goal to work towards and is a nice way of improving your entire town rather than just your own home.
There's no problems with the game like indestructible items as far as I can tell, but supposedly grass deterioration is still a slight issue.