I've only done one road race now, so this is not really a fair comparison, but it gives you the insight of how it felt like to do a road race from a mountain bikers perspective.
Mountain bike racing and road racing are very different. For example, in a mountain bike race I mostly go my own speed, you often hear a mountain biker saying "I rode my own race", i.e. the fastest speed you can do for the length of the race without bonking, so it's like an offroad time trial. Only if I have a target to chase - someone in my eyesight, which is a lot shorter distance than on a road race, I usually go out of my comfort zone to push super hard to catch and overtake that person. Or say I know somebody to be a good technical rider, then I try and go hard to stick to that person on the uphills because once on the downhills I can try and follow his line which can often make you go faster. The average speed in a mountain bike race is between 10km/h and 20km/h, depending on how hilly and technical the terrain is. You don't always find somebody matched closely to your speed on all types of terrain or even just the same speed up as downhill. Actually, the best races are those where you do find somebody who is closely matched to your speed and ability, so you have a good battle all the way!
Due to the faster speeds in road racing, a road race is much more group oriented, you try and stay with the group to be sheltered from the wind (except when you are trying to break away), so you are automatically more dependent on the speed of the pack or otherwise you'd be a loner against the wind all by yourself. The difference of riding by yourself on the road or in the pack is huge: I was averaging almost 38km/h in the road race due to being able to "hide" in the pack. When I go on a lonely road spin on my mtb, my fastest spin ever for about the same time was averaging 28km/h - a whole 10km/h less and it felt a lot harder than when I was riding in the shelter of the pack!!
The other outcome of this is of course that you ride sooooo much closer together in a road race - it's a lot more "intimite". Yes, it happens in a mtb race that people run into each other, or when a person falls in front or you and you crash into them, but in a road race it's almost elbow on elbow and wheel on wheel! I felt it when a guy pulled in a little bit too early just in front of me and touched my wheel. In mtbing you never have more than 2 or 3 people closely around you apart from the start and that only for a very short period of time, i.e. when you overtake them or they overtake you. I've had many races where I felt like I was going around the course by myself for long stretches of time!
The start was also different. In a mountain bike race the first lap is the one were people sprint off like the devil is behind them and go super hard to get into the single track first. So after a lap in a mountain bike race I am usually already totally wrecked and I try and recover in some of the following laps while trying to hold my overall position and to gather energy for the last lap. In a mountain bike race the first lap is usually the fastest and the last one the second fastest (or at least faster than the one just before). In the road race we didn't even really start racing until the 2nd lap.
And also, I find a 2 hour mtb race is a lot harder than a 2 hour road race. On an mtb race there isn't as much "chilling out" as in a road race, without the possibility of hiding in the bunch. You always go hard and even though you have downhills, these are often so technical, that you can't really chill out that much - you might not be able to pedal because you have to hold balance with the use of your upper body, but not pedalling often means you can't clear the accummulated lactic acid easily, which can lead to cramps once you go hard again. If the descents are not technical, you pedal through them. I have had a lot more lactic acid in my legs on a 2 hour mtb race than on this road race.
Another difference is that in a mountain bike race it also seems to be more a case of you against the course - every lap is challenging and in each lap you get better at choosing the right line down a tricky descent - there is so much more dramatic change in scenery and terrain per lap. The road race seemed a lot more monotonic and a matter of you against the other riders.
And finally - a nice thing about road racing is the minimal preparation before and minimal clean up after. In a mountainbike race I would turn up the day before to pre-ride the course and to decide on which tyres to use and organize feed zone support, whereas in a road race you just turn up 5 min before the start and that's it! Similarly, while post mountain bike racing there is bike washing, muddy gear, collecting bottles from the feedzone etc. whereas after a road bike you just put your bike into the boot, throw on your jeans and you're done! Wow.....
Any roadies out there who would like to give their perspective on a mountain bike race? I would love to hear what roadies have to say about doing one of our races. There's one more chance left this year: the last Irish Mountain biking NPS will take place on the 28th of September in Carlingford. Any brave souls out there who want to give it a try? I'd advise to pre-ride the course the day before if you haven't really done much off-roading. Don't be put off with the technical sections though, you can always get off the bike and walk those bits ;)