Server worlds can get impressive pretty quickly. One of my wallpapers is this render from one of the old reddit server worlds: http://i5.minus.com/im5gOe.jpg
On the other hand, my single player world which is about 4 years old isn't nearly as impressive.
I migrated my single player world (dating back to Beta 1.8) to a server so some of my coworkers at the time could play too. It's not really huge, but I am strangely emotionally attached to it. My server was down for a few months (and I wasn't playing at the time), but I recently brought it back online. Minecraft seems like one of those games that I'll play forever, like Super Mario Bros.
On the other hand, my single player world which is about 4 years old isn't nearly as impressive.