Decimate/un-subdivide turns quads into diamonds? This is my mesh before subdividing:



After subdividing x3:



After un-subdividing x3:



The diamond topology mess up smooth shading and make the edges look terrible from certain angles.

It might seem like there's no point in subdividing only to un-subdivide, but the purpose is to get the smoothness that subdivisions give, but still retain a low poly count. How do I obtain to a low poly count, but keep the smoothness of subsurf and decent toplogy?
This happens when doing multiple iterations. If you un-subdivided once 3 consecutive times you would keep the same quad layout.

Try it. Subdivide a mesh three times. Then click spacebar, search un-subdivide, click it. Now click spacebar again, search un-subdivide etc.