Demonstrated stability and control with only thrust vectoring (and gyros?) - no aerodynamics. Validating that their control loops respond fast enough, no weird oscillations, they can deal with noisy real-world environments and inputs (not clean simulations in the computer lab) etc. Verifying navigation back to a fixed point (ok, they didn't seem to translate any, just up and down). Verifying sensors, control, and engine throttling for hover and soft landing. This was actually a fairly comprehensive test.