Jeffrey Wang wrote a great blog post about attribute overwriting in MDX that is very clear and full of helpful pictures to show what happens when you write an MDX statement that writes into your multidimensional space. This is very common in an MDX Script and if you tried to customize the DateTool solution you probably experienced how hard this concept can be.
The point is not that MDX is hard, is that a model based on multiple hierarchies in a dimension (and each attribute is a hierarchy by default!) have many hidden complexities for calculations.
This is a very good reason to say that DAX is simpler. When you are used to MDX, DAX seems harder, but in reality this is not true. It’s different and, from a certain point of view, it doesn’t have the same expressiveness of MDX. But the example provided by Jeffrey definitely makes it clear why so many people scared by DAX are looking with interest to PowerPivot (and attend our workshop, too!).