The many-worlds interpretation of quantum mechanics seems to
have been developed with extreme carelessness, as far as I can tell.
Suppose the universe is a one-dimensional harmonic oscillator
in an energy eigenstate. That's an extremely simple quantum
state for the universe to be in, it should be easy to 'interpret'.
So: if that's the global quantum state of the universe, where
are the many worlds? What are their states, their histories?
Frank Tipler (in _Physics of Immortality_) advances himself
as a many-worlds advocate. When he tries to describe what the
many worlds *are*, at one point he says they are *all* the
trajectories through the classical state space. At another
point he refers just to the Bohmian-mechanical trajectories
through that state space, those corresponding to a particular
choice of universal wavefunction.
You can see more of my complaints about the MWI at
under "Challenge to many-worlds advocates".
According to John Bell, at one time physicists would say that
Niels Bohr solved all the problems of interpretation. I think
that Everett is starting to play that role - people who have
doubts or puzzles about what QM "means" are referred to Everett.