2.1 will feature 'near' infinite flat worlds.
The maps will be 512 blocks high.
You will be able to control the ground level just like you can now.
You will be able to control the horizontal size to one of the following:
256 x 256
512 x 512
1024 x 1024
2048 x 2048
4096 x 4096
8192 x 8192
16384 x 16384
32768 x 32768
A new flat type called Natural will also be added. This will have grass on top, then the usual dirt, clay, sandstone etc layers down to rhyolite and bedrock. Just like a flat survival map would be, but without caves and ores. The ground height is not changeable with this type.
The reason to only release flat worlds initially is to get infinite worlds to the public but also give us more time to improve the terrain generation. Because once the terrain generation is released, you will likely need to create a new world if we go and change the terrain again, so we need to get the terrain as close to what we want it to be on its first release.
Releasing only flat worlds first will also give us an opportunity to iron out any kinks caused by infinite worlds before releasing them proper, kind of like an infinite worlds beta.
And finally, I don't want future updates to take up to a year again, so if we have a big feature, releasing it in stages like this is a way to mitigate that problem.