I came across
this article about Common Core mathematics tonight. It's wonkish but well worth the effort of reading in its entirety. A quote:
...there has been a de facto national mathematics curriculum for decades: the curriculum defined by the school mathematics textbooks. There are several widely used textbooks, but mathematically they are very much alike. Let’s call this de facto math- ematics curriculum Textbook School Mathematics (TSM). In TSM, precise definitions usually are not given and logical reasoning is hardly ever provided...because the publishers mistakenly believe that intuitive arguments and analogies suffice....
From this perspective, the main point of this standard is that these calculations with fractions are taught in the fifth grade. Indeed, the very purpose of mathematics standards (prior to the CCSMS) seems to be to establish in which grade topics are to be taught.
By the same ridiculous token, if a set of standards asks that the multiplication table be memorized at the beginning of the third grade or that Algebra I be taught in the eighth grade, then it is considered to be rigorous.
These are just bits and pieces. The author has a lot to say about how to teach math in a more meaningful way and deficiencies in teacher education in mathematics. I don't agree with everything he wrote, but the article is very thought-provoking and encouraging about the Common Core.
Here are some newer follow ups:
This one is by the author of the article quoted above.
A math ed. blogger 
