I really like the fact that minecraft is an infinite world, bus sometimes you may want a world that doesn't just go on forever and keep creating new chunks. I think that when creating a new world you could have the option of making your world finite or infinite. Some other options for finite worlds would be a world size(large, med., small, and maybe custom), and being able to choose a regular "flat" world with actual limits, or a world more like a globe. For world creation in general I think some other useful features are determining your starting biome, and maybe even the land type, like hilly or flat, and maybe the ratio of water to land.
I think some configuration options when creating a new world would be great. Mountainous, flat, sea level height, general biome, ect. I am sure that this will come along eventually. Max map size I like as well(SMP mainly). keeps someone from just walking west for an hour.
http://www.minecraftforum.net/topic/14470-wrap-around-planet-like-worlds/
Mostly moved on. May check back a few times a year.