On the first screenshot we can see how smooth lighting (both on minimum and maximum) looks in minecraft 1.7.2, with hard edges, on the second picture, its actually smooth, back in minecraft 1.4.7.

What I was expecting to happen:

Smooth lighting to look smooth with no hard edges, like in 1.4.7 and before

What actually happened:

Smooth lighting is broken, and has been like that since minecraft 1.5

How to reproduce:

Use the old versions of the launcher to go to minecraft 1.4.7 and see how the lighting is different.