Other differences to consider, road bikes are geared higher, have bigger diameter tires which also have a higher psi then the mtb tires causing less rolling resistance and are a lot lighter.

You could put some semi-slicks on, change the rear cassett to a road cassett (i would leave the mtb cassett on though since both road and mtb cassetts have 11t cogs on them and you would still have the hill climbing 32t or 34t cog), use a larger big ring up front and run higher psi in the tires. the mtb still wont be as efficiant on the road as a road bike but it will help. And with the semi-slicks you will still have some grip for your off-road excursions(sp?)