One big issue with XBRL presentation relations in general and the US GAAP Taxonomy in particular is the vagueness of the "parent-child" relationship which is used to express relationships.
Basically, the arcrole "http://www.xbrl.org/2003/arcrole/parent-child" used to communicate that there is in fact some sort of relationship leaves open to interpretation exactly what that relation is and what it means. While what is expressed might be clear to those who use the "parent-child" relationship to express something; the intent tends to not come through, be misinterpreted, be inconsistent because of different people working on different areas of a taxonomy, and in general leads to confusion.
Providing some general examples helps one understand why this is a problem. The document A Taxonomy of Whole-Part Relations goes into detail. This presentation (on slide 9) explains the difference between a taxonomy and a partonomy and provides these general categories of relations:
It is pretty easy to see that if the relations in something like the US GAAP Taxonomy were made explicit that the intent of the creator of the taxonomy would come across more clearly, the discipline of having to follow an explicit scheme of communicating relations would lead to better consistency, and understanding and using the taxonomy would be significantly easier.
Further, extensions create by say the 10,000 public companies which file with the SEC would be easier to interpret if relations where explicit.