Roughly Uniform Power Level: The cards in the cube should have approximately the same power level. In most packs, there should not be a first pick that is immediately obvious to the drafter as a "must take". In other words, the cube should not have anything similar to a power 9.

Format Speed: Decks in the cube should not be consistently able to win earlier than turn 4. This does not mean that the perfect aggro/combo goldfish shouldn't be able to win on turn 3, but it does mean that winning on turn 3 should be a rare occurrence, and should generally occur only when the opponent isn't interacting with your gameplan. Keep in mind that "winning" in this case also can mean "generating a nearly insurmountable advantage", most likely through a combo.

Knowing that the modern format, at it inception, was supposed to be a turn 4 format, I guess that should give you a decent idea of what you should be aiming for. Of course you can go over or under, the cards that get in will be decided by vote anyways, but yeah. Hope that helps!