The Cartan subalgebras of a reductive Lie algebra are abelian.
Are there non-reductive Lie algebras with abelian Cartan subalgebras?
In fact, the elements of a Cartan subalgebra of a reductive Lie algebra are semisimple, so a weaker question is:
Are there non-reductive Lie algebras with abelian Cartan subalgebras all of whose elements are semisimple?
N.B.: I asked this earlier at math.SE.