Sliders, and lots of 'em
I'm no longer an avid player of Minecraft but the world customization feature planned for update 1.8 has grabbed my attention. Here's a video demonstration to get us started.
Through the use of 16 sliders (or direct value input), you'll be able to tinker with the game's terrain generator. The variable that's got me excited is sea level. And the fact that there's a "randomize" button is always encouraging -- I usually mash on that in character creators.
Plus, presets. Mojang says these should result in worlds "we think players will find interesting, including a world made of water up to the top of the map, and the return of floating islands."