Since everything so far has been mostly negative, can anyone think of something good Davis did for the war effort?
Because of his military experience he knew, by reputation if not personally, most of the generals on both sides. This made him a fair judge of competence, if not character. Most of his command choices were decent: Beauregard, A.S. Johnston, J.E. Johnston, Bragg, and of course R.E. Lee. In contrast to the first set of Union generals, who were a very mixed bag, the first set of Confederate generals were generally competent.
On the minus side, he tended to let personal animosity color his decisions and occasionally interfered with operations to an extent not seen again until Vietnam, with equally poor results. His appointment of Lee as generalissimo in 1865 was four years too late; had he let Lee direct operations from the beginning while he stuck to being President the Confederate war effort would have been much better organized.