This is a loaded question. It's difficult to predict historical outcomes, but I think that if popular sovereignty had become law the United States will likely have been divided into two pieces. The Confederate states may have formed their own country, and the North may have continued on in its own fashion, since the South was trying to secede from the Union.
In terms of which issues should be left up to the states, this is another really tough question. I believe basic rights and privileges of a democratic republic should be upheld everywhere, including abortion, voting, freedom of speech/ religion/ personal expression, and gun ownership (this is not an all-inclusive list). The states should be left to decide how to allocate their resources, such as for education, law enforcement, economic development/ resource allocation, taxes, etc. (again, this is not an exhaustive list).
I must also state that things are more complex than can be efficiently stated in this book discussion, and that I'm sure there are many issues of which I am significantly naive about!