What is the Crystal System of MoS2?
Naturally occurring molybdenum disulfide can occur in either of two crystal systems, hexagonal or "trigonal"; the latter is also known as rhombohedral (see here).  

In both cases the coordination geometry around molybdenum has only threefold rotational symmetry, but in the hexagonal form the crystal will add a center of symmetry to become hexagonal.  Such a conversion is seen in other cases such as hexagonal close-packed metals like magnesium and zinc, which actually have only threefold symmetry about any axis perpendicular to the basal plane and passing through an atom, yet the Crystal's are hexagonal.  In general, when you have a unit cell with $D_{3h}$ symmetry -- a mirror plane perpendicular to the threefold exis -- you will get hexagonal crystals in both molybdenum disulfide and hexagonal close-packed metals.

The rhombohedral crystal, with a different arrangement of mirror planes (the point group symmetry is $D_{3d}$ instead of $D_{3h}$, and $D_{3d}$ has no mirror plane perpendicular to the threefold axis), has the required center of symmetry without going from threefold to sixfold rotational symmetry.