As zippyRN says, it's difficult to directly compare the US system to the UK. We are a fair few years behind in the implementation of nurse practitioners and physician assistants (although we do now have them).
Being perfectly honest, senior nurses are capable of a lot (probably more than junior doctors). What distinguishes doctors though is the capacity to make decisions. Nurses tend to work within rigid frameworks and guidelines and the moment something a little different happens, they require leadership and guidance. For sure, they possess the clinical skills and for some, even the knowledge and aptitude; however; that is not the key role of a doctor within a multi-disciplinary approach, for example.
Secondly, I believe that responsibility sets the two apart. Whilst nurses are in charge of direct patient care, they are not generally responsible for the patient. Again, this is a decision making process and similarly to above, doctors are not constrained to conventional thinking. They are regularly forced to try and think 'outside the box'.