1. Your bike is different.
As a mountain biker, you are used to putting your back into it when you need to lift the thing onto a bike rack, over a log, or so forth. My own preferred method is to use the 'Clean and Jerk.' If you use similar force when lifting a road bike, there's a good chance you'll accidentally throw it over a building.
Also, you need to pump the tires up harder. Much harder. No, even harder than that. Generally, in fact, it takes the weight of two or three 'roadies' (an endearing term road cyclists like to call themselves) to push down hard enough on a standard floor pump to get the tires to the proper pressure.
How do you know when a road tire is inflated to the proper pressure? The answer is simple: it's hard enough when one single more stroke of the pump will blow it off the rim. The real art is, naturally, in knowing whether you've reached that point.
2. The terrain is different.
When you are mountain biking, you naturally are inclined to look for interesting obstacles to ride over -- roots, rocks, fallen logs are all part of the fun. On a road bike, on the other hand, anything but perfectly smooth pavement is a potentially life-threatening danger, and must be avoided at all costs. Further, if you are ahead of another cyclist, you must use elaborate hand gestures to indicate that there is -- horrors! -- a pebble 75 metres up the road.
3. Words you know have different meanings.
Since roadies and mountain bikers have a common heritage, it's no surprise that they share some vocabulary. It's also no surprise that the variance in meaning in some of that vocabulary can get you into trouble.
For example, if a mountain biker says a ride is 'technical', you can assume that there is loose shale, several ledge drops, high-penalty (e.g., death) exposure on one side of the trail, or slick, mossy roots twisting along the singletrack. If a roadie calls a ride 'technical' on the other hand, it most likely means that there is a roundabout somewhere in the ride.
As a second example, when a mountain biker talks about going on a 'group ride', it means that a bunch of friends got together, regrouped at junctures of the ride, talked as they were riding, and probably had a beer or twelve together after the ride. When roadies have a 'group ride', on the other hand, riders are expected to ride in a tight formation, paying strict attention to the gap between your front tire and the rear wheel ahead of you. the gap should be no more than four inches. After the obligatory ten minute warmup, it becomes each rider's dual purpose to drop every other rider, while not being dropped yourself.
4. Beware of triathletes.
As a mountain biker, you've always been deeply suspicious of triathletes. As a road cyclist, you will find out you were correct to be so, and you will find out why. Triathletes will try to infiltrate your ranks and join your rides, then demonstrate that they have no idea of how to ride in a group, and very little control of their direction of travel.
Most importantly, though, they wear these short shorts and tank tops that are just plain creepy.
5. You must now keep your bike clean.
On a mountain bike, dirt is a badge of honor. A little mud on the downtube tells other riders that you're not afraid to ride in the rough stuff. On a road bike, on the other hand, if your bike isn't 15 percent cleaner than when you bought it, you are a slovenly ne'er-do-well who cannot be trusted.
6. Your body needs to change.
As a mountain biker, you've no doubt noticed it's quite helpful to have not just strong legs, but strong arms as well. Roadies, on the other hand, regard their arms as a necessary evil, their sole function being to keep their chests from falling onto the bike's stem.
It's a well-known fact that roadies bind their arms to their sides when not riding bikes, doing everything they can to facilitate the atrophy of these non-contributing limbs.
7. What you look at changes.
When mountain biking, you have no doubt been astounded at the beauty around you -- the trees, the streams, wildlife, beautiful sandstone vistas. As a road cyclist, you will also find yourself occupied with things to look at, such as the pavement. Or, if you're riding in a group, you'll be treated to the constant, unavoidable sight of the butt of the guy riding ahead of you. And cars flying by you, yelling out helpful suggestions about what you should do and to whom, as well as their understanding of whether you belong on the road (their stance is that you do not).
It's breathtaking, frankly.
