I've done exactly this - having broken an old MTB frame after 2500 km on it, I decided to buy a road bike.
Searching and researching and browsing suggested 105 level components would be a good floor, and I had a preference for a disc brake on the front. Pricing worked out around $2200 NZD for an aluminium and about $2500 for the same thing in carbon.
Instead I bought a 1998 road bike for $100 from trademe (the local ebay) and its great. I ride further and faster.
You'll end up riding 5 km/h faster on average, and your climbing times will drop by ~10-20%.
Answer: Comfort makes for a good road bike.
Comfort comes from a good body position, so bike fit is more important on a road bike than a MTB. On an MTB you're up and out of the saddle a little more, on a road bike you maintain a fairly similar position a lot.
Smoother tyres help - you can go full slicks on a road bike, but it will be terrible off sealed road (gravel roads are not fun) and a patch of gravel on the road can surprise you.
Road bikes have hard tyres too - tube air pressures are 100+psi and make for a hard ride. In a new bike I'd look for steel or carbon forks, to take the rattly edge off the ride. My aluminium forks are "buzzy" and that leads to hand numbness.
Consider hanging out for something with Brifters (integrated brakes and shifters), if that's what you want to try. Downtube shifters were good back in the day, but you're trying ideas for a new bike - nothing has downtube shifters any more.
I suggest you look for a good used road bike, ride it for some months, and refine your expectations and requirements. I still want a front disc brake, but it can wait.