You will find the differences are significant.
When you say a road bike, we understand a bike with drop handlebars. Drop bars give you a very different riding position compared to an MTB. Some people need time to adjust to the new position, and can have problems with weak or tired neck muscles.
An alternative would be a hybrid style of bike, with flat handlebars. This would be much closer to the riding position you are used to.
The thinner, smaller, slick tires will provide two more differences.
One is much less cushioning, so you'll feel bumps more. The other is they'll be much faster. So you'll ride faster and feel any bumps in the road more.
As a consequence, some people get sore hands or wrists.
The other main difference is how light the bike is. On any medium quality bike (or better) you'll feel like you suddenly got on an F1 bike :-)
When switching to road cycling, make sure you have well padded gloves to absorb road vibration. And the bigger the change in riding position from your MTB, the more gradually you should make the change. Keep the MTB and ride the new bike for a few days, then switch back. If you start feeling sore then give the new bike a break for a day or two.
It may sound like I'm talking road bikes down, but as a road-only cyclist I am keen for you to get the best out of the change. You'll quickly discover that each kind of cycling has it's own different pleasures.