Spawned on a beach, right next to a mountainous forest. Peaks and valleys and some very large trees gave the idea of a treehouse. Not much exploration done yet, but some plains are nearby. Possibly towns, strongholds or mines as well.

This was my first Minecraft world. Back then, it was a quiet, snowy little place. I have a cottage, tucked away by a frozen lake. It was very peaceful to have a fire in the fireplace and watch the snow drift down. Since the updates it rains rather than snows. And without exploring too far there is a decided lack of resources available.
Still, it's a nice house.

Mountaintop castle, tower near spawn, underwater base, some fairly extensive mineshafts. Not the prettiest but it was a lot of fun to make.

There is a Nether pathway between the underwater base and the castle. It's mostly protected from ghasts. Mostly.

I've been working in this world for a bit and some of the terrain is older, but there are some very interesting features that make it neat to explore.