A5: The major problem for beast master rangers is that the beast master gives up Prime Shot. Prime Shot gives access to the Prime Shot feat tree of Called Shot+Improved Prime Shot(i.e. +2 to hit/+5 to damage at a cost of 2 feats - that's really strong). Also, as you can't take Archer Fighting Style, you can't qualify for Battlefield Archer, a really strong Archer paragon path.
At the same time, though, Twin Strike is really good and Sharpshooter isn't a bad paragon path.