How much of a difference will I notice going from a Mountain bike to a road bike?
You will find the journey a lot less effort, as you are dramatically reducing the weight of the bike.
You will also be able to accelerate much faster, as the rolling resistance will be reduced through the use of thinner tyres.
Your gearing is likely to allow for faster speeds as well.
If you previously had suspension, you will notice the ability to accelerate, brake and turn is much better, and more precise.
You will rapidly learn how uncomfortable bumps are so you will have to get used to dodging round stones, potholes etc.