Chunks (16X16) of the world have failed to load, apart from loading the lowest flat level of the sea bed...
The first area on survival I've found as a savannah, it means everything should go right...
I did load this world READY FOR 1.7.2, but clearly hasn't rendered properly.
Screenshots!
This is not a rendering error, but rather an example of a chunk border between 1.6.x generation and 1.7.x generation.