Well, for one, the combatants would have essentially no control over their own movement in zero-G. They may be able to push themselves off from other objects (including allies and opponents), or learn to flap a cloak to "swim' through the air, but rotation is going to be hard to deal with.
Physically hitting something hard enough to do damage would be very hard. Swings are bound to miss and/or do little damage. Stabs may work a little better. Archers would be much better off, although they would need to learn to control the rotation caused by firing an arrow which is not aligned with their center of gravity. Spellcasters are probably best off in zero gravity (may need a concentration check for tumbling and/or simply for being in zero-G).
In d20 terms, Nimbleness is Tumble and Athletics is Jump.
- You can move along the walls of the craft by making an Athletics check DC 10 or a Nimbleness check DC 20.
- All movement that is not along a wall occurs in a straight line and you continue to move the same distance each round until you interact with another object.
- You can push off a wall by making an Athletics/Nimbleness check. The distance moved is equal to the result of the check (rounded down to the nearest 5ft.).
- You can always move 5ft. This is as a 5ft. step that consumes your move action.
- You can move by throwing objects in the opposite direction. Throwing an object of your size causes you to move your speed. Throwing an object one size category smaller moves you at half speed and throwing objects larger than you causes you to move at double speed.
- Attacking with a melee or ranged weapon causes you to become increasingly unstable; the first attack has a -1 penalty on attack rolls, the second -2, and so on, to a maximum penalty of -5. A full round spent stabilizing allows you to begin again at -1.
- Range increments of projectile weapons or abilities are doubled.
- You can not be tripped or knocked prone.
Also, if flight is a possibility:
- extraordinary flight (flapping) has reduced maneuverability
- supernatural \& magical flight still works the same
- levitation requires that you specify an axis as up \& down
As I come up with new/better, I'll post in a stream-of-consciousness style.