Minecraft's real star is its landscape, flowing psuedo-randomly from whatever name you give your world. But it also became a checkerboard of predictable components: rolling hills here, weirdly-shaped mountains there, and perhaps an abrupt patch of swamp or tropical jungle between them.
But not anymore: a new update, out today, revises the land-making algorithms and adds a bunch of new biomes--areas with a distinctive climate type, flora and fauna--and creates more natural transitions between them. There are cliffs, giant lakes, canyons, redwood forests, all sorts of new flowers and grasses, the option of wildly eroded "skylands", as pictured above, and much else besides.
A lot of other things are also improved in the "snapshot" preview of Minecraft 1.7, including a far more elaborate fishing system: you may now find all sorts of things in the water. This Reddit thread has all the details. (Note: If installing the snapshot release, it'll create ugly seams in saved worlds anywhere that the old meets the new)